I've always thought it was crazy to have the umpire standing in the middle of the field like an inside linebacker. Apparently the NFL has decided it's crazy too. From ESPN:
Pereira said umpires -- who traditionally have been positioned on the defensive side, 4 or 5 yards from the line of scrimmage, just behind the linebackers and between the tackles -- were knocked down by players more than 100 times in 2009. Two umpires suffered concussions and three needed surgeries for knee or shoulder injuries.
There will still be three officials on the defensive side of the ball. I'd imagine it's going to take some time to get used to watching in reverse, but it doesn't seem like it should cause a problem. The two main duties of the umpire are to call holding (which would seem easier when standing in the offensive backfield) and watching for offensive lineman downfield illegally. The second one would seem harder to enforce when standing in the offensive backfield, but it's not a frequently called penalty anyway.
This may cause a problem on a play or two next season when QB Aaron Rodgers scrambles in a wide arch. With an official behind on either side, it's possible he's more likely to run into one of them. I wouldn't expect it to happen often, but maybe it will happen as often as you used to see a wide receiver running into the umpire on a crossing route.