I agree, Celtic have it all to do, but if they can (somehow) get a lead at Celtic Park, maybe just maybe they could hold out at the Camp Nou. However, I still think Barcelona are too good, and they may well have a new coach by then with Rijkaard looking under alot of pressure (remember it is Barca-Real this Sunday, so he could be out by Monday!).
I don't expect United or Chelsea to find themselves in an insurmountable position in the return legs, but it would be prudent not to take either Lyon or Olympiakos lightly. Lyon are seasoned CL performers and have Benzema, while Olympiakos have the feel good factor.
Arsenal and Liverpool have the hardest tests though in the Milanese giants.
The Gunners, if they can play that expansive football they have been this season, I feel can beat AC Milan at the Emirates, and maybe even at the San Siro. However, Milan are European (and World) champions and have more than enough in the locker to punish mistakes ruthlessly.
Liverpool-Inter is the tie of the round though, and very hard to call. Rafa's men are a Champions League team whether they like to admit it or not, and Inter are stronger than they have been for a very long time. Unfortunately for Liverpool, the second leg is in Milan and I believe this will ultimately be to Internazionale's benefit.
Looking at the other games, Schalke-Porto could swing either way but Porto have far more experience at this level.
The biggest possibility for a shock might be in the Fenerbahce-Sevilla tie. Fenerbahce look like a very strong opportunistic team that respect no one (and let's no forget a certain Mr Carlos, and Deivid isn't bad either!), while Sevilla's lack of CL experience could begin to tell.
And finally, Roma's tie with Real Madrid looks to be a fairly safe bet on Madrid, but Roma did knock out Lyon last season when they weren't backed to, so perhaps Totti & Co could cause problems. The first leg at the Stadio Olimpico will be vital.