After paying the $10 to rename your account, does the old gamertag point to the new gamertag, or is the old name released back into the pool for anyone to claim? (Looking for something definitive—Yahoo Answers hasn't been much help here.)
It appears that the gamertag is released back into the pool of available gamertags.
I changed mine a couple of months ago. Just now I logged into a different Xbox Live account and attempted to change that account's gamertag to my old gamertag, and I got the message that my old gamertag was available.
I also tried to send a friend request from this second account to my old gamertag, and I got the message "The gamertag you entered does not exist on Xbox LIVE" which further confirms this.
Microsoft is also releasing gamertags that were previously claimed and "dead" for one reason or another. They don't say why specifically, but I imagine gamertags associated with banned accounts are among those that may get auto-released.
As far as "aliasing" is concerned, when you change your gamertag, everyone who has you on their friends list will see your new gamertag in place of your old one, immediately. It's up to you to let them know that you have a new gamertag (or just watch them get confused when you play together, like I did... :P)