I have an late 2010 MBP 13", core 2 duo and upgraded to 8Gb ram, 256Gb SSD. I've noticed the same behavior since I upgraded to Moutain Lion 10.8.2. The cursor seems to Lag sometimes (sometimes it looks like it's freezed - just like when the batteries are dying), and sometimes it went back fast. I talked to Apple Support here in Brazil and they are suggesting to downgrade the OS to a lower version than 10.8.2 and wait for a fix to upgrade. I've read in other forums that sometimes it's related to a older version of Dropbox installed (upgrading might get fixed - didn't worked for me though). Hope to get a fix soon!!
It happens to me with any wireless mouse since 10.8.2, but specially with magic mouse. It just become unusable, with strange lag all the time until I reboot. However, it returns suddenly to lag.
Note: the trackpad works fine while mouse is lagging.
MBP early 2011, with 8gb of ram...
Waiting for a fix
I had the same issue. I bought an IOGear bluetooth dongle and used it with a USB extension base
Works FLAWLESSLY. Even during intense gaming when both the Keyboard and mouse would get so laggy it was impossible to play. It even works better than it did during normal use when I didnt have the display plugged in.
One thing to check, OSX directions sucked, so do this.
1. Leave your bluetooth on, with all devices still configured/connected
2. Option Click the bluetooth icon on your menu bar
3. Write down the 'Address' listed. This is the address of your internal Bluetooth
4. Plug in the Extension and then the Dongle.
5. Wait a couple seconds
6. Option click the bluetooth icon again
7. Make sure a different address is shown. If so, you are now transmitting/recieving from the dongle and not the macbook.
8. I plugged the extension into my Thunderbolt display and now I can just unplug and go. All peripherals will still work as it autoswitches back to the internal bluetooth once disconnected.
Hope this helps