Ok, Mr. DJ Traveler, my two cents. I have never been married or have a girl cheat (that I knew of) so I have no idea what you're going through. I have been "josh" a couple of times, though.
So for what its worth, here's my advice:
you marraige is over. you will not recover any trust.
Number one priority is to protect the kid. He is your responsibility. I mean that if she gets custody, he's gonna have a daddy of the month (see Bible-belts 450 necklace thread). so do what you gotta do to get custody. Let her cheat just so a judge will give custody to you. the keylogger has been the best advice thus far.
Number two priority is protect your wallet. Make her leave you first, if you can. that sounds evil and wicked, but all is fair in love and war. first it was love, now you are at war. nothing less. The future of you and your child is at stake. Your wife gave up all rights to both of you when she chose to do what she did.
Number three, if its possible, is after you ditch the b1tch, get a new wife that is more mature and will be a decent mom to your kid. That is more important that some girl that will give props.
and lastly, and this sounds totally and awfully fvkked up coming from a complete stranger, but thanks for posting. Those of us lucky enough to not be in your crappy situation can learn from your mistakes.
Godspeed brother, I mean that.
Taiyuu