Garageband for the iOS works great with bass guitar. There are many different fine companies who make adapters for guitar/bass and the iPad/iPhone/iTouch.
Garageband does not have a bass specific amp. However the "clean" amp sounds pretty good. I usually use my Sansamp between my bass and the iPad. It helps to beef up the sound.
There are thousands and thousands of other music apps besides GB that will make an iPad a great addition to your "studio". Too many for me to go into. If you ever considered experimenting with synthesizers, the iPad is a great jumping off point.
You are only limited by your imagination. There is a bunch of stuff I have experiment with on my Soundcloud that I used my iPad2 and Garageband for. Take a listen. https://soundcloud.com/greggbummer