We've got the details on transferring tracks from Rock Band to Rock Band 2. A just-released title update to Rock Band will add a tab reading "export disc" to your main menu. Click it. Follow directions, and you'll have 55 tracks from the original Rock Band playable in Rock Band 2.
There is no free lunch, however. The transfer costs $4.99 in Microsoft Points. But don't worry about the memory too much; word is, the transfer will only eat 1.52 GB of your drive.
Three songs from the first game will not be making the cut, however, due to (we imagine) licensing and rights issues. Those tracks are: "Paranoid" as made famous by Black Sabbath, “Run to the Hills” as made famous by Iron Maiden, and "Enter Sandman" by Metallica.
Metallica is a sad loss, but the other two tracks are covers, and "Run to the Hills" is a bear to play, so, overall, it's 5 bucks well spent.