On the import side, players will once again be asked to pay a fee to export the songs from the Rock Band 2 disc. If you've already purchased export keys for Rock Band, AC/DC, etc., you'll see those songs immediately in RB3. There may be some omissions, due to licensing, in the RB2 setlist when moving to RB3, however.
As for Rock Band 3 DLC, all of it will support Pro drums and Pro keyboard for no additional charge. Due to the amount of extra work involved for Harmonix to chart Pro guitar and bass, it will run you an additional $1. It was either that or "include [Pro guitar] (in the regular download) and up the price for everybody, which I don't think is really fair," according to DeGooyer.
Not all DLC songs will get Pro guitar and bass charts as many bands' "guitar parts wouldn't rise to the level that they would need to have pro mode authoring associated with them." However, "one major band that we're working with that has already got some DLC out there," will get new DLC and have their old DLC retrofitted for Pro mode. It better be Rush... just saying.
Additionally, there is "a selection of other tracks in the [Rock Band] catalog that we've gone back and identified" that will also get Pro guitar and bass support, explains Degooyer.
Rock Band 3 will be released on October 26 for the PS3, Xbox 360, and Wii. A DS version will also be released.