Macbook Pro won't sleep when plugged in

Ever since I installed Lion, my 2011 MacBook Pro won't sleep when it's plugged in. I choose 'sleep' from the Apple menu and the display and keyboard go dark, but the status light stays lit (it doesn't "breathe") and the Macbook stays warm. If it's not plugged in and I choose 'sleep' then it sleeps right away (breathing status light). This only started happening when I installed Lion. although I didn't notice this right away so it could've started a few days later and be coincidental. I've tried the obvious stuff (I think) like rebooting, turning off all sharing and bluetooth, holding the power-button down for 5-seconds when off, etc.

When I look in the Power Management Logs I see lots of ''Domain: wake" entries but no "Domain: sleep" entries for the past 3 or 4 days, except for when it's on battery power.

Any ideas? Thanks!!!

Here's some system info:

Model Name: MacBook Pro

Model Identifier: MacBookPro8,2

Processor Name: Intel Core i7

Processor Speed: 2.2 GHz

System Software Overview:

System Version: Mac OS X 10.7 (11A511)

Kernel Version: Darwin 11.0.0

Boot Volume: Macintosh HD

Boot Mode: Normal

Secure Virtual Memory: Enabled

64-bit Kernel and Extensions: Yes

Time since boot: 3 minutes