I've got one of the '07 Fisher Caliber cross-country bikes, which I dearly love. For the 2008 bikes, Fisher replaced the Race Day frame they used on the Calibers with the HiFi frame, essentially the HiFi is replacing the Calibers though you can still order the Race Day frame if you want to rebuild yourself a do-it-yourself bike.
I test rode one of the HiFi 29ers, and liked it. The big difference I saw between the two was the asymmetric chain stays on the HiFi, which look a little wierd, but seem to work at least as well as the symmetric stays on the older bikes. I stuck with the Caliber, though, I just liked it a little better overall, plus the sour-apple green color was kind of kewl compared to the pale-looking HiFi.
Where I bike, it's mostly flat with short, sharp rises, so it's debatable whether I get the full benefit of the dual suspension. I do enjoy it on a couple of local trails, though...