Well, since the cap is based on a percentage of revenue, and revenue is definitely going to be down, I'd say yes, the cap will shrink. This won't affect the C's much because they are not in a position to be under any kind of cap for a long time.
This year's cap is around 58 mill. Celtics payroll is at 80 mill this year, 74 mil the next. In the '10 offseason, the Celtics Payroll drops to 44 mill...if they let everyone except Perk, Pierce and KG go. You have to figure Rondo gets an extension, raising payroll to at least 50 mill, and if the cap shrinks, that could conceivably give the Celtics about 4 million in room with 4 players on the roster. Not going to happen. They'll probably extend Powe/Davis, pick up options for Giddens and Walker, as well as add rookie salaries. Bye bye cap. The only way the Celtics will be adding players is via trade. Fortunately, if teams are really worried about the cap/tax, then Ray's '10 expiring contract starts to look really enticing. I'd much rather the C's use that to get some younger players this next offseason than they just sign him to an extension. It stinks, but the C's will really hapve to try to move Ray this offseason, in my opinion. He's going to be too old to extend after that and no one will really want him in a sign and trade at his age. The C's could extend him cheaply, but then there's no way to add impact players and that's an old roster. As I said above, I really think the C's will move ray and take back 1 or 2 younger players with longer contracts. It's the only way for the C's to add younger impact players, and if other teams are worried about payroll, it could be possible.
Where it hurts the C's is the luxury tax. If the tax lowers as well (I don't think it should, it seems unfair to suddenly drop the tax line on teams), then there's less motivation for the C's owners to maintain current payroll, therefore meaning less incentive to replace Allen's big contract with other, long term deals, meaning they'd be more likely to let him walk to stay under the cap (meaning we'd be looking at a roster of Rondo, Pierce, KG, Perk, Giddens, Walker, Powe/Davis, a MLE free agent and one more rookie or so, with Pierce and KG being around 34 or so), or just extend ray cheaply, at about 35 yrs. old.