First off, I'm not sure why people assume Mottram is the better swimmer. Lance was one of better distance freestylers in Texas in high school and Texas is a swimming powerhouse. Also, while Mottram's 19:55 1500 meter swim is repectable, it is certainly not stellar. A sub 20 1.5k open water swim is really not that difficult as long as you have some sort of swimming background. Give them both time to train in the swim, I'd call it even at best for Mottram in the swim.
As for the bike, this leg comprises about 55-60% of the race for the Ironman elites, making it the most important. Lance would absolutely destroy Mottram on the bike. It would not even be close. Mottram's 1:01:50 40k bike split at Noosa is rather pedestrian even for age-group triathletes. Now, Lance is a seven time TDF winner and former World Cycling Champion. That's comparable to a 16:00 5k runner racing Paul Tergat in the marathon. Craig Walton put 6 minutes on Mottram in the Noosa race, and even though Walton is probably the best cyclist on the ITU circuit, Lance would easily be able to put a few minutes on Walton (when in shape). Pro cyclists can go sub 50 in a 40k TT. Now expand that over 4 times the distance where Mottram will rapidly deteriorate in efficiency and strength whereas Lance would be able to keep close to the same pace.
A 112 mile bike ride is a brutal challenge, even if you just want to finish it. Now try racing it, then tacking on a marathon afterwards, Mottram would crumble. Remember, Lance rode 112 mile bike rides daily during his career. In the TDF he raced 100+ mile races daily for a whole month. After completing a 112 mile bike race Lance would be immeasurably more 'fresh' than Mottram, who may have never even ridden a bike ride that long before in his life. Mottram may even struggle to break 3:00 in the marathon depending on how tight his quads/hamstrings/calves are and how much pain he has in his lower back from the bike. Being a triathlete, your running ability can be tremendously mitigated if you don't have the aerobic base on the bike to be able to handle the long rides and come off the bike feeling loose and ready to transition into a run. Lance obviously has more than a 'base' for cycling. Because of the fact that the bike comes before the run, Mottram's superiority in the run would be lessened.
Mottram averaged just over 24 mph in his 24.8 mile bike tri race. Normann Stadler, who won the 2006 Ironman, averaged over 26 mph for a 112 mile ride. Assuming an in-shape Lance can atleast hang with Stadler, and more than likely put 10-15 minutes on him, and giving Mottram a generous 22 mph average, that's already over 50 minutes Mottram would have to make up. Lance would probably pull off a 3:15 marathon or so; there is no way Mottram is running a 2:30 or under.
Also, for those who say Mottram couldn't race in the Noosa Elite wave because they didn't want him to win, which may have been the reason, in reality there is no way he would have been close to winning anyway. In fact, his time would have probably been the same. His swim was over 1:30 slower than that whole Elite pack ahead of him which would have made it impossible for him to catch up to and draft off that bike pack ahead of him. So he would have ridden solo anyway. In ITU races, you can't win the race on the swim, but you can most definitely lose it. Even in an olympic distance race, Lance could probably pick up 8 minutes on the bike, and given Mottram runs his 32 10k (which I expected to be much much faster), I'm sure Lance would be sub 40.
So that is my outlook on the race.