First, let's start with the guarantees:
Mail, Calendar, Contacts, Find my Device. These were specifically called out as being at the core of iCloud.
Then let's wander to the questionables:
Websites, MobileMe galleries, iDisk, Back to My Mac.
Of these, I have to assume Back to My Mac will stick around. It's a new enough feature, and has a very 'cloud'-esque nature to it, plus it's built in to the OS and the AirPort line. Doesn't really use any Apple resources, either, so it seems like a no brainer.
iDisk, websites, and galleries all look *VERY* questionable at this time.
My gut feeling is that galleries will be transitioned to "Photo Stream". Not the same, but that will be Apple's claim of continuity. (Which is too bad, because I have a *TON* of galleries shared for friends and family.)
iDisk seems like it is going to be replaced. The new 'doc syncing' feature seems to be its replacement. Yeah, again, not the same. No arbitrary-file support, no (yet-announced) public folder system. I figure it will probably be like the iWork.com beta (which I ALSO figure will be done away with in favor of doc syncing,) where you can choose to make individual files public, and they get their own URL. Again, not the same as an OS-mountable FOLDER, but that's my gut feeling. I figure it won't be long before some enterprising developers create an app that will allow arbitrary files to be stored and shared, though.
Websites I'm the most uneasy about. Nothing even remotely similar has been announced as part of iCloud. iWeb hasn't really been updated in quite a while. ("iWeb '11" is really the same version as in iLife '09 - and isn't even available from the Mac App Store - only if you buy it on DVD. Same with iDVD, which hasn't even been really upgraded since iLife '08.) My gut tells me websites are going the way of the do-do. And, with the emerging popularity of Facebook, I can kind of see why. iLife has always been about home users, and 'personal websites' have really tanked in popularity as Facebook has risen. Of course, I also know plenty of small businesses (including many small Mac and iOS software devlopers!) who use iWeb with MobileMe hosting for their websites!
Also, now that I think about it, the rising popularity of Facebook is a likely reason for phasing out galleries. People now share pictures and video on Facebook (or Flickr, Picasa, YouTube, even Twitter, etc,) so Apple probably figures by tying in more direct connections to those, they don't need to have their own similar storage location for public content. This realization further increases my belief that galleries will be going away. "Replaced" (at least that's what they'll call it,) by Photo Stream.