I had the same problem... but since my education basically was dealing with training cats and dogs... I fixed it. :)
First, I don't know why people thumbsdowned the suggestion of a screen door. If you aren't in the room, a screen door allows fresh air, your ability to hear your baby isn't dulled, and the cat stays out. It's not a bad idea, and I've seen it work a LOT with folks who have stubborn cats or who can't take the time to train.
For our cats, we kept a screen over the crib when the baby wasn't in it. They sell these nettings that keep bugs out, and this can keep cats off it as well. We also kept a blanket over the entire crib mattress just in case the cat DID get on the crib's bed, you could take that off and the crib was clean for the baby.
Next we taught the cats to leave a room when told to. Basically, this was simple. You get meat-baby-food (less salt and not bad for cats, every cat I know will take it. If yours doesn't, try meat spread. Avoid tuna.) Then you basically give the yummy outside of the room at a designated spot (say a cat bed, or a cat-tree.) While there, say the word "tree" or "bed" or whatever you want to call it. You could make a certain noise, or blow a whistle, click your tongue, whatever. After they figure out that when you say that word/noise you give a treat in that spot, start taking them away from the spot and making the noise, then going and giving the treat. Cats usually pick this up really fast, and even when I was 8 we trained our cats to run to the bathroom where we would lock them for the night (they'd get out of the house otherwise and our neighbor killed our previous cats, so containing them at night was for their own safety.) Being able to tell the cat "scat" and have it run to a specific place really makes it easy to make sure the room is cat free.
There are inaudible deterrents, but I don't recommend them because I am not sure how safe they are around babies to be honest. But if you want to get high tech... there is the electrical scat mat you can leave in the crib when the baby is not in there. This will give a static charge when the cat makes contact with the mat (not super ouch and since it will deter when you're not there... MUCH better than a water bottle. Besides, you don't want the crib to get wet.) Third link.
I also like the SSSCat sprayer for those cats who are particularly stubborn. Set it up at the door-frame of the room and when the cat comes within a meter of the sprayer, it will send out a blast of harmless air, and scare them to run away. I didn't have to get to the point of using the electric mat or the spray can.
Do make sure your cat is vaccinated though. Just in case. Better safe than sorry. You can always look at softpaws as well if your cat seems inclined to scratch the baby. But really, I just taught the cats to stay out of the room my daughter uses, and we keep the door closed when she slept and used a baby monitor (with the activity level in the house, it kept her asleep better.)
Hope this helps. :)