My own personal opinion is that Valentino Rossi will not return to MotoGP this season. And the reason why he will not return has nothing to do with the broken tibia he suffered last weekend.
Lost amid all of the speculation of how much resemblance Rossi has to Superman (will he get back on the bike in 2 months, as some optimists believe?) is a potentially more serious medical issue for Rossi. To wit, the shoulder injury that he suffered over the Winter riding a Motocross bike.
On May 21st Yamaha finally publicly admitted what was already common knowledge in the pit lane in MotoGP, that the injury was affecting Rossi's ability to ride his bike at the maximum. Here is the MotoGP lead doctor and his Yamaha crew chief:

"The situation with his shoulder has always been more serious than anyone thought," Costa told Motosprint magazine.
"First of all, the shoulder almost dislocated in the crash, and this has caused an instability, albeit a limited one.
"He has an inflammation of the bursas: what is known as periarthritis; there is a very important tendinitis at the long head of the bicep; and the cartilage is damaged in the area that completes the skeleton of the shoulder blade [glenoid cavity]. It's an important temporary disability."
Yamaha crew chief Jeremy Burgess says the injury has been slowing Rossi down in recent races, where he has been overshadowed by team-mate Jorge Lorenzo.
"The telemetry is clear regarding the way Valentino is forced to ride these days: he is forced to enter turns more slowly, he has to adopt an inefficient position on the bike and sometimes he rides one-armed, in order to rest the other arm," said Burgess.

This is a collection of injuries that would probably have folks like me wearing a shoulder sling and undergoing intense physio and rehab... the important thing to realize is that these kinds of injuries heal relatively slowly.
The cynic in me (shock horror. Moi?) believes that, following the principle that injuries are often under-reported by athletes in order to avoid giving information to their rivals, Rossi might have more serious issues with the shoulder that can only be cured by surgery. For a MotoGP rider, whose body has to work quickly and effectively to shift the bike around a lot, a deficient shoulder is a bad impediment.
Even minimally invasive shoulder surgery has a significant recovery time - up to 4-5 months. I would not be surprised if Valentino Rossi also undergoes shoulder surgery in the next 2 months. If he does opt for surgery, his 2010 MotoGP season is over. Which would really ignite the speculation that Rossi may opt out of MotoGP to race in F1, although it is not clear how he could competitively do so, since there is no agreement in prospect to allow teams to run three cars, the only obvious way in which Rossi could race for Ferrari. It is difficult to see Rossi signing for any other team, unless it is a team that uses Ferrari engines, and neither Sauber or Toro Rosso are exactly setting the F1 world alight right now.
My guess is that Rossi will end up taking the rest of the season off, partly to heal his leg but also to heal his shoulder, which could end up taking longer to heal than the leg, and will consider his options over the Summer.