It appears compatibility is no longer an issue. Taken from GH:WT's wiki:
"Activision also stated during their 2008 E3 press conference that for Xbox 360 users, the existing Rock Band instrument controllers as well as other third party controllers will also work in Guitar Hero World Tour, though it is unknown if Rock Band 2's new instruments will work. Compatibility of instrument controllers on other consoles is not yet known.[8] When non-World Tour instruments are used, the game will adjust the tracks to allow them to be playable; for example, World Tour normally presents drum notes along five different tracks to the play in addition to the bass kick pedal line; when Rock Band' drum set is used, two of the tracks will merge as to provide four such tracks.[9] It has also been said by Neversoft director Brian Bright that the drum set for World Tour would also work with the first Rock Band, although he was unsure of whether they would work with Rock Band 2. This currently applies only to the Xbox 360 versions of the games."
Well, at least to the lucky 360 owners of the game. In short, this means that if you own the 360 version, you only need to buy one game's full instrument set, then get the other game. Probably the GH:WT set. It is currently unknown whether the IONAudio Drum Rocker (Rock Band 2's premium drum set, at a large sum of $299) will work with Guitar Hero: World Tour as a 5-note set, but I'm sure some mod-God on the internet will come up with something.
Well that's always a plus. Now I can just buy the game disks only for the next games.