WiiFit will be, at most, an odd growth off the market, sort of like a benign tumor. It'll swell up and just sit there for a long time, as housemom's buy the WiiFit, use it for a week and then let it collect dust along with the treadmill, thighmaster, abcruncher, and any other gadget the blew good money on in order to lose 20 pounds. This will not get the majority of them into gaming, and will have far less effect on the market than, say, WiiSports (which is, at least, an actual game).
At most this will create a weird VG offshoot of such peripherals, but it will be rare for the two markets to crossover. At most you'll get a symbiotic relationship in which the gamer buys the console, the housemom buys the gadget and usurps the Wii for an hour a day, and then the gamer will buy games that use the gadget (WiiSurfing or somesuch). This will have the effect of increasing overall Wii sales and putting money in Nintendo's pockets, but other than increasing userbase, the gaming market shouldn't be shaken too much (unless companies go blind with jealousy and abandon games for WiiFit knockoffs).