Unfortunately you cannot import tracks into GarageBand on the iPad, it seems.

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I really think GB on the iPad is amazing, but yeah, this one is a biggie.
But all is not lost :
Apple said in the keynote that GarageBand iPad projects would be compatible between the iPad and Mac version.
I tried to open an iPad .band file into GarageBand '11 on the Mac, and it told me that it was created with a newer version and wouldn't open it.
So I guess that GB on the Mac will get an update soon to add compatibility with the iPad version.
In the mean time, I think I found a way (a hack) to import external audio files into the iPad GarageBand. I haven't tested it yet, but I don't see any reason why it wouldn't work.
-Create a blank song in GB on the iPad.
-Record some audio on one or more tracks.
-Save the project and send it to iTunes.
-Copy the .band file on your HD using the "Application" tab in iTunes.
-Go into the Finder, do a right-click on the .band file and choose "Show Package Contents"
-Open the Media folder, you'll find one or more .aif file(s).
-Convert the tracks you want to import as 16-bit 44.1Khz AIFF files using Quicktime Player 7 or some other tool.
-Change their name to exactly match the name of the files in the .band package.
-Replace the .aif files from the Media folder with the tracks you want to import.
-Copy back the .band file to the iPad using iTunes.
It's an incredibly convoluted way of importing audio, but I wouldn't be surprised at all if Apple added audio importing in a .1 release, they did similar feature updates to Pages, Numbers and iBooks not that long after their initial release.
At worse we'll be able to import audio through Mac created GB projects once it has been updated to be compatible.