Kill It With Fire!
Right, we're going to get Medieval on this problem.
This is bad troubleshooting technique, because we are going to address all the issues at the same time, and not bother with locating the heart of the issue (because there really isn't one to find, it's just MobileMe).
Firstly, let's get up to date. Go to System Preferences>Software Update>Check Now. Get all the updates that are available.
Once that is complete, go to System Preferences and ensure that you have the MobileMe panel. If it still says .Mac we need to click on it, and under Account, Sign Out. Go back to the System Preferences window and wait 5 minutes. You should get a pop up about updating MobileMe or the such, choose yes to what it offers you and allow it to update.
If you don't get offered the update, don't worry, run software update again and see if anything new is there.
If nothing happens then, let's carry on without the update. We need to reset all the synced data on MobileMe, so ensure that your computer has all the data you wish to sync present and correct.
Go to System Preferences>MobileMe (or .Mac) and enter your details again. Then go to System Preferences>MobileMe (or.Mac)>Sync and at the bottom of the window, Advanced. Choose to Reset Sync Data
Go back to the Sync window and firstly ensure syncing with MobileMe is set to Automatically, and then press Sync Now. It will take several minutes to send all the data to the MobileMe servers.
You should now have clean data on MobileMe and your computer will sync correctly with MobileMe.
Now, to correct the iPhone issue. We have clean data on MobileMe so the issue lies on the phone now. I recommend going to Settings>Mail, Contacts, Calendars>MobileMe and choose to delete the account.
Once that is done, turn the phone on and off, then go to Settings>Mail, Contacts, Calendars and add the account again.
Wait up to ten minutes for your data to be added to the phone. A wifi connection will speed this process up.
If the problem still persists, ensure you have updated to 2.1, and test it again. If the problem is still present, restore the phone to 2.1 and restore your user data at the end of the restore process.
Finally, if this still hasn't resolved the issue, there may be a corruption in your user data and that is the source of the problem. Restore the phone, but don't put your backup onto it. Start fresh by choosing to set up as a new phone.
This is a monster post, so please read carefully, and don't hesitate to ask for clarification.