This probably won’t matter to most people out there, but I thought it was worth mentioning.
If you use aliases with your .Mac account, and you have an iPhone, be careful who you reply to. An email sent to your .Mac alias can be received just fine on your iPhone, but if you reply from your iPhone, your REAL .Mac address will be used. The iPhone can’t send from your alias, like your desktop app can.
Hopefully this will be fixed in a future software update.