Hmm... I bet it was Saturn. Ring-a-ding-ding!

I normally get pulled into some form of pattern by external forces (work, class, appointments and the like) so I don't drift as much as I otherwise would. Still can vary by up to about 1 - 3 hours for the same consistant morning alarm time.
I haven't seen a sleep specialist. I don't really think it's that out of the ordinary - I remember seeing on a doco years ago that it's not abnormal to drift an hour or two per day. I'm just... at one end slightly stretching the definition of "normal"

It's not as bad as it seems. I don't often get the oppertunity to let my body clock do it's own thing. I've usually got to be up at some time for some reason.
I've also heard some meds can mess with sleep patterns. Makes sense - it's all neurochemistry. I've also heard the same link with depression and sleep disorders. It might be worth looking into if you are experiancing issues.
No such explanation for me though. I'm not afflicted with the curse of depression.
The "twilight zone" (Oh yeah, I sure know where that is. I have a condo there

) can pull you in if you aren't careful, and it can be difficult to get out again. Especially if you're depressed at the time.
I cut loose when I have holidays and follow my own body clock into the twilight zone willingly (it's nice not to wake up tired!) - but I try to stay no longer than a week at a time. It's just too easy to stop interacting with the world when it starts looking like disjointed time lapse. I also wouldn't go near it if I were feeling depressed - that sounds like a sure fire way to send yourself nuts.
The best tools for dealing with the "twilight zone" (living in it, or leaving it) are a firm grasp of 24 hour time, and clocks that shove the time and day / date in your face. I switched when I was a teenager because I got sick of not knowing if it was am or pm (and not knowing if the day had elapsed yet, or how many days had elapsed). Just having the real time and day of the week in your face can help you stay connected with "reality" enough to bring yourself back when it's time.