I have Dropbox on both my desktop and laptop. I don't want my desktop's wallet.dat anywhere near my laptop.
Occasionally I'll log into my Dropbox from someone else's computer, to pull a saved document. With my wallet on dropbox I would be even more worried about them having pasword-capturing spyware.
And even assuming I trust everything is secure on my end, I must then trust that staff (and contractors ?) at Dropbox who have at least read-only access to storage won't scan for all wallet.dat files?
Now instead of linking to the wallet.dat, I would periodically create an encrypted copy of the wallet (wish that were automatic or part of the client) and then copy that to Dropbox. Then, to get that to Dropbox on Windows I'd use something ike MS SyncToy.
That works too, if you have a lot of bitcoins I wouldn't put it there unencrypted either.
It's just a matter of how experienced you are with all the tools and how much bitcoins are at stake.
I hope the bitcoin client will have some crypto for the wallet sometime in the future, as it's a big risk if you get hacked, and the more "money" that are out there the bigger the motivation for hackers to try get it