Not a bad idea if you want to run fatter tires and have fenders, at the same time. And if you don't mind dealing with a smaller selection of wheels (rims anyway, if you build your own) and tires.

I have a 650b bike (not a conversion) and I can't say that I've ever been bothered by the lack of selection on rims and tires, although I guess one misses out on bargain priced 700c rubber sometimes.

Concise guide on doing this from the same guy who designed your bike: http://www.rivbike.com/kb_results.asp?ID=64