that is a good question. the last person i would trust in the office is michael. i saw that episode today too and i was wondering. good question goalstopper.

The only reason he told micheal was because he loves Pam, and theyd just announced their wedding date, so Jim was very depressed and needed someone to talk to so went to his last resort (other than Dwight).

Maybe alcohol gives Jim a loose tongue. I have that experience at parties where people that I'm only vaguely friends with get drunk and feel like they need to talk and for some reason I'm always the one they decide to open up to. It's baffling but it happens a lot.

I think he just needed to vent,
she just set a date for her wedding and had just broken up with kaity,
he needed someone to talk to,

If I were Jim, I probably would have done the same. There have been times that I was depressed and I talked to someone about it that I never thought I would have.. And Michael was extremely nice and kind-hearted to Jim, in that episode anyway. In the episode entitled "The Secret", Michael is back to being Michael..

How about the writers wanted to create some suspense or tension in the plot and telling Micheal was an easy way for the word to get back to Pam. So Jim tells Pam it was an old crush and Micheal slips that it was a recent crush thereby creating more push and pull between Jim and Pam that concluded with Jim's revelation on Casino Night.