I have an H1-B and was worried about this when I first used it (and this applies with other visaa also). You get an initial I-94 to go along with your visa when you enter the country.

I have found that if you leave the US to Canada, they will not remove the I-94, and it is ok to re-enter the USA with this one. INS should not hassle you over this.

If you leave via an airport to an international destination (e.g. Paris France), the airline should collect your I94 card at the last point prior to USA departure. When you re-enter the country from an international destination, you will need to fill out the I-94 card to be placed into your passport.

There is one technicality, if you leave the US-Canada-other destination, they will not take your I-94, though you are supposed to surrender it. Usually they do not have a problem if you retain it.

Also, I had to do a new I-94, get fingerprinted and photogaphed entering from the Pacific to LA. I do not think this is normal, but a painless procedure overall.