The 16 (assuming Lampre gets their papers in order) remaining protour teams from last year will be in because of the agreement between ASO and UCI. Cofidis & Bbox have been assured of an invite as well, that makes 18.
They would surely invite Radioshack since it has LA and a pretty strong team overall. That's 19.
If they stick with 20 total then they can only invite one out of the following teams:
Cervelo
BMC
Sky
Vacansoleil
Skil-Shimano
Saur-Sojacun
I'm pretty sure they want last year's green jersey winner and an ex-winner so I'd say Cervelo is in. I also think they want BMC for displaying the WC as well as having a serious contender (2nd twice!), so I think they'll go for at least 21 teams. And if they decide to go to 21 they might as well go to 22?
They may opt for Sky to fill the final spot due to Wiggins, but since the rest of the team isn't exactly a great GC squad I think they'll go for Vacansoleil who have the Feillus and a team that has shown it's willingness to animate races by attacking frequently. Skil-Shimano even though they did well in last year's race don't seem like they have anything special to add so they'll surely be out. The same goes for Saur-Sojacun: even though they're french the competition for entry slots is so fierce this year that they must pick a better team - perhaps this favors Vacansoleil even more since they have french riders.
In short: I think Sky, Skil and Saur are out. In case another team drops out due to doping scandals (or in case Lampre doesn't manage to extend its license) then they'll probably pick Sky.