Last summer my wife and I has a series of garden parties/potlucks.  While we encouraged folks to bring their own reusable tableware, for the slackers we bought some unbleached fiber plates and compostable, I think corn, plastic utensils and cups (sorry, I don't remember the brand).  These materials were added to the compost and subsequently covered as the pile built up.  We also added some plastic containers for dates, strawberries, etc. that were labeled as compostable.  The compost was not turned, just built up in layers in a one square yard footprint slatted plastic compost bin.  I turned the compost in the first part of June and sifted finished compost.  All the plates and most of the cups and containers were gone, but the utensils were still there and looked basically pristine.  I'm not sure how many years it will take for them to break down.  It seams like a commercial or community-sponsored composting program where all the material is chipped up would be a better disposal option for this type of plastic.  

I remember being super stoked when I first heard about biodegradeable corn plastics.  However, my enthusiasm was brought down several notches when I looked further into it and discovered that at least some corn plastics are made from genetically modified corn.  In fact, it seemed as if this was a deliberate effort by those that would raise/market/promote genetically modified corn to "green up" their image.  (I don't have the research on hand at the moment, but much of it was available online about a year ago.)  That being said, I hate to tarnish the entire corn plastic industry since there may be some less nefarious outfits not using GMO corn.  However, if it comes down to choosing between petroleum-based plastics and GMO-corn ones, seems like it's time to just cut down on our plastic use entirely, and re-use and recycle what we do use as much as possible.

(I also seem to remember a lot of hype about biodegradable plastics in the early `90s, but it turned out (if memory serves) that they were held together with non-biodegradable plastic polymers--when the biodegradable component broke down, the plastic polymers were free to leach into the ecosystem.)

A couple of comments on the comments:  

There are several ways to make biodegradable plastic.  One is to use biologically-based polymers that may or may not break down in a composting facility or a landfill.  Another common technique is to use mostly petro-plastic, but insert bio-plastic monomers into the plastic as it is made.  Plastics made with these hybrid polymer chains will break down into smaller pieces quickly, but will still leave tiny particles that will not degrade.  I call this "out of sight, out of mind" plastic.

There are also different kinds of landfills.  The "dry tomb" design tries to isolate waste, and make sure nothing gets in or out.  Another type uses soil between lifts.  Depending on the content of air and organic matter in the soil, aerobic degradation may occur.  Anaerobic degradation (which is much slower) occurs in many landfills.  The end-product, methane gas, is often captured and used to produce energy.  

But there is no guarantee that degradation won't occur in a dry tomb, or that it will occur in the other types.  Bill Rathje once studied landfills across the USA, and found no consistent pattern of degradation.  He found 60-year-old newspapers with the headlines still readable in some landfills, and paper turned to mush in others.  Local climate had less of an effect than he anticipated.

The best solution is to do what matrogers did: At an event where you typically use disposable items, do your best to use reusable ones.  The first 'R', 'reuse', is always the most effective, because the use of the second (reduce) or third (recycle) always results in the fifth: residual.

As a sales/consultant to the foodservice industry, let me say that the bio-based or BPI (Biodegradable Products Institute) certified products are a huge growth industry & heres why.
1. In-house cafeterias of heathcare facilities schools & universities are lowering their solid waste tipping fees by implementing a waste diversion program utilizing bio-compostable plates, cups, containers,cutlery & unbleached dispenser napkins that are made with
PLA (corn) resin instead of #6 polystyrene
switch grass, sugar cane stock & talc
pre-consumer milk carton waste scrap
post consumer recycled paper
These bio-compostable packaging items are then thrown into bins & roll off containers along with compostable food scraps and taken to commercial compost facilites such as the Intervale here in Burlington Vermont. It is processed the most efficient way possible which is commercially. The Intervale profits from this nitrogen-rich compost created by a mix of BPI certified bio-compostables & food scraps. The healthcare facility enjoys a 70% savings on waste disposal costs through this diversion process. The savings pays for the compostable packaging.
BPI certified products which adds credibilty and guarantees there will be no contaminates into the compost heap.

Conclusion: Corn based plastic is only the 1st generation resin used to make some of these products. There will be other biobased resins made with soy bean oil, palm oil, sunflower oil etc.  There are also biobased commercial cleaning products made with these non-petroleum oils to form the base detergent system to a green cleaning product.

My advise is to leave the composting of these bio-plastics up to the commercial facilities, not your backyard composter. Use reusables at home & when youre in a takeout establishment that uses disposables such as syrofoam (polystyrene) or polyethylene coated paperboard, recommend they look into bio-compostables as an alternative to the petroluem based packaging that are BPI certified as that criteria means the item fully degrades in a commercial composting environment.