I got a copy of Rock Band 2 Special Edition for the Wii for Christmas this year (thanks to my beautiful wife, Missy!). The Special Edition bundle includes the game disc, the drum set, the microphone, and one guitar.
The one other guitar that I already own, an "official" Guitar Hero 3 guitar that was packaged with my copy of Guitar Hero 3 for Wii, does work successfully with Rock Band 2 for Wii! It "just works" -- no special configuration or setup was necessary. So with the Rock Band 2 Special Edition set plus the Guitar Hero 3 guitar, my family has been able to play Rock Band 2 with the full four-person band (singer, lead guitar, bass guitar, drums).
Really excellent job by the folks at Harmonix for taking the time and effort to make the Guitar Hero 3 guitar compatibility work, and also for making the Wii version of Rock Band 2 feature-equivalent to the XBox 360 and PS3 versions! Thanks, guys!