Personally, I would set it up as imap.mail.me.com
I imagine the address gets distributed to individual servers based on availability and load.
The letter-number combination is likely a server location and numbering system to help with administration, so you end up with servers with names like p99-imap.mail.me.com and p07-imap.mail.me.com or something like that.
It seems that lots of the mac.com and mobileMe users got assigned directly to the p99 server. And when that server ran into problems over the last couple days, it effected many users.
If those user's accounts had been set to imap.mail.me.com their traffic would have been re-directed seamlessly to a functioning server and they wouldn't have noticed there was any problems.
In essense, the "automatic" setup is the only thing that failed when it assigns you directly to p99 instead of the controlling server imap.mail.me.com