I wouldn't bet on Nintendo never licencing from Sega, they already have some first party Sega games on the Wii VC (I beleive), and Sega have no interest in the home hardware market any more so they probably wouldn't say no if the price was right.
However the Dreamcast is a fairly powerfull system for pure software emulation, and the games would be a problem (would have to be distributed via physical media due to the size), so it's very unlikely we'll see it on the Wii.
However, if the Wii's VC is a sucess (and it's looking quite possible), then I wouldn't put it past Nintendo to try and improve it on the next console down the line to add additional console compatibility (either via hardware - I suspect the DC's specific hardware could be added to a modern chip relatively cheaply*), or via clever software.
The bigger problem would be MS, as the DC used some windows variant and I suspect it would need a fresh licence to use it.