Lets get a few things straight here ladies, and try to draw the lines about the trust part.
Its OKAY to feel you trust your guy still if he talks to his co-workers and you're usually invited to their group events as his partmer so you can also see the kind of chemistry your guy exhudes around each one. You will feel alright if everythings clean, but if/when you feel a pinch, you should trust your instincts, and its not always your guys intention to stray, women can get pretty manipulative too in order to get what they want.
If your guy is unnecessarily chatting too much on msn or facebooking more than he should, even if he tells you everything, DON'T think its OKAY. this is the difference of where is the appropriate place to trust and where not. In Career/Social life you gotta deal with people, but on internet we usually choose ourselves who we want to befriend.