I think any runner that is seriously curious about this question should invest in decent bike, a USCF license, join a good local bike club for a year or two, and find out for themself. After I retired from running after the 1988 Olympic Trials I did exctly that. I joined the Northeast Bike Club of Bedford, MA. Bike racing is really fun. But it's also more dangerous than running. I met folks there who had just as much knowledge about physiology of exercise as most of the runners I'd known. Some trained "high miles" (350+/week, like I did) and some alot less. After a couple of years I was able to place high enough in the New England USCF Time Trial Championships to go to the Masters National TT Championships in Borego Springs, CA where I was soundly beaten by dozens and dozens of better riders.
A few reasons I eventually returned to running:
The amount of free time I have in my day. I found that to get the equivalent of a one hour run I needed to spend about twice as long on a bike. If you are married with children then you know how precious that hour can be. Maintenance time needed to keep your running shoes in top mechanical condition: almost none. Unless you have enough money to employ a technician or a GREAT sponsor who can staff the team with one, plan on devoting a decent amount of time to keep your bike tuned up perfectly.
Mechanical difficulties. The physical demands on the body during competition I found to be very similar. However, having to DNF in a bike race because of a flat, or another mechanical difficulty when I had invested so much into the training and I had paced myself well the first 3/4 of the race was frustrating.
Injuries? The myth that bike racers don't get injuries is bunk. Ask a cyclist how many times he's missed a season due to a broken collarbone or the like. (I had one.)There are plenty of injuries to cyclists, some are even fatal. To both runners and riders, be careful out there!!!
The team aspect. The team stuff on bikes can get pretty political sometimes. I brought a pretty darn good cardio system to NEBC when I came, and as a result had some decent early success in the pack races (crits and road races). But even a good novice rider should plan on having to "Pull" during the race (let other more senior team members draft behind them, thus conserving their energy), not to mention "lead out" in the late stage of the race. This is done by letting the team's best sprinter draft behind you from say a mile before the finish line, you go as hard as you can until they slingshot past you to the win. You have to pay these "dues" before you'll get your crack at the top.
As the title of Lance's book sugggests "It's not about the Bike" you don't need the most expensive bike to beat riders of similar quality. But I'd have to admit that if you don't have a pretty good bike for things like Time Trial you won't have a chance. When I went to Nationals the fastest guys had great bikes and more importantly the man and the machine were one, perfectly syncronized.