I think anyone that lives out of town that you would reasonably expect to attend your shower (close family and friends) should get the same invitation that everyone else gets. If you're not planning to invite them and/or it's obvious they wouldn't be able to attend, don't send an invitation at all. I would not send a notice of a shower- do you notify people about other parties they aren't invited to? I would feel bad if I received something that said, hey- I want you to know about this cool party I'm having that you're not invited to- want to send a gift anyway?
I'll add that there is an option for out-of-towners who *would* be invited if your baby shower were geographically convenient to them. If the subject of the shower comes up, I'd say, "Oh, I just hate that you're so far away now! Of course the only reason we didn't mail you an invitation is that I know it's a long trip, but you know that if you happened to be in town we would love to have you!"
I have lots of close friends who fall into this category - mailing an invitation is too gift-grabby, but since they will hear about the party through the grapevine, I make an effort to help them feel included without an obligatory-gift request.