What types of plastic bags are recyclable at grocery stores?

Excepted from a post on CafeMontgomery (a local treasure trove of all sorts of community information!):

...what type of plastic is allowed in the recycling bins at the grocery stores?

I was just cutting the paper labels off some clean, dry bags, and realized that they feel very different from one another, and that some have numbers and a recycling symbol and others don't...( I found 1, 2, 4, and 5!)

...so please, tell me what I should be doing!

Many of the collected grocery bags end up going to Trex, a company which turns grocery bags, stretch films (like the kind used to wrap loaded shipping pallets), and waste wood into recycled plastic decking, railings, fencing, and landscape edging.

Here are the Trex specifications for the grocery bags they accept: http://www.trex.com/Suppliers/grocery.asp

In a nutshell, they want bags made from polyethylene, which is resin #4.

A quick tipsheet for interpreting those other resins mentioned in the specifications:
PVC: resin #3
polystyrene: resin #6
polyurethane foam: if labeled at all, should be resin #7
polypropylene: resin #5
PET: #1

Thanks, M. -- great question!