I noticed this immediately after upgrading from Snow Leopard, to Lion. I'm using the exact same iMac as you.
I've tried changing out the batteries, jogging the settings in both the dedicated Mouse prefs and the Universal Access prefs (there's some more mouse prefs buried there). I've also cleaned any debris from around the optical sensor, made sure there were no physical obstructions preventing the mouse from clicking down. Everything checks out with no problems.
There's definitely something either in the Bluetooth transmission-receive or strictly in the software/firmware causing this.
I use my Mac 8-12 hrs a day and this mouse problem is really hurting productivity. Apple, if you're listening, some swift response and action would be appreciated. Thanks.
I'm seeing the same problem - left mouse button either "sticks" on pressed or won't press.
I have a 15", late 2008 MBP with a wired external keyboard and a bluetooth mighty mouse (model A1197). I've seen that other folks attribute this problem more w/ a bluetooth issue than anything else.
Funny thing is that for me, it didn't happen until I updated to OS X 10.7.3. Not on 10.7 or 10.7.2. Go figure.
I use my MBP 8-12 hours per day. The left-button lock-up seems to resolve itself for some period of hours after a reboot - which I end up doing 2-3 times per day. When the left button on the mouse misbehaives, the MBP trackpad exhibits the same behavior. the keyboard still works fine, so I often end up using as many keyboard short cuts as I know to try to shut stuff down, and have to occasionally resort to the press-n-hold of the power button to reboot.
The boards are covered with anecdotal fixes and I'm currently at a loss to find a root cause or official Apple solution. I'm reluctant to downgrade back to OS X 10.6 or to revert my mouse bluetooth drivers back to the stable (pre-10.7.3) versions.