What is the format for Xbox LIVE member profile URLs?

The format was changed to 'http://live.xbox.com/en-US/Profile?gamertag=[Gamertag]', where '[GamerTag]' is your personal and unique name.

So your example would now be: 'http://live.xbox.com/en-US/Profile?gamertag=Xonatron'.

The new format is https://account.xbox.com/en-us/profile?gamertag=gamertag.

It still requires a login but after that the format works.