You can listen to a preview of your purchased music to help you identify a track before downloading it (when browsing by album) but you can't stream complete songs.
Can you use Mail with a custom domain, or does it have to be me.com?
You can use Mail with any mail account you want. But iCloud's e-mail system is based solely on the me.com domain, so far as we can tell.
Will parts of iCloud be accessible with a browser?
We don't know yet, but we'd assume that you'll be able to at least view your mail, calendars, and contacts (as you can with MobileMe now). The stuff that requires iTunes (apps, music, books) probably won't work in a browser, although photos might.
I have a few albums I copied from a friend. How will iTunes Match know which files are legal for me to upload?
The specifics of how iCloud does or doesn't deal with pirated music will likely be a secret. We'd guess that if you try to match tracks purchased from someone else's iTunes account, you'll be told that you don't have the right to use them. Those tracks are watermarked with another Apple ID and therefore easy to identify. As for tracks that have been downloaded from the Internet, it's possible that Apple could look for identifying characteristics that indicate the music came from a dodgy source. For example, tags, watermarks, and sonic footprints. On the other hand, it's possible that part of the $25 you pay each year buys you a measure of amnesty from the labels.
Can you delete apps through the cloud from all devices by deleting on one?
It doesn't look like there's any facility to delete items on other devices from a single device. Which is nice, because it might make the other people who share your Apple ID quite angry.
Can I choose what syncs and what doesn't, per device?
The iCloud settings area suggests you can turn contacts, calendars, reminders, bookmarks, notes, Photo Stream, Find My iPad, and iCloud backup on and off individually on each device.
With "the truth is in the cloud" in the new OSes, what about people who want local copies of everything (e.g., POP vs. IMAP)?
It seems like Apple's solution here is to push everything back out to all the Macs or PCs attached to your account. But if you're uncomfortable with having the canonical form of your data be on Apple's servers, iCloud may not be for you.
What happens to data when you delete a game and reinstall it? Is there a way of restoring game saves?
It seems likely that game saves will be syncable using iCloud, so you can pause on an iPad and pick up on an iPhone.
Will using iCloud's photostream on the Mac require me to upgrade to iPhoto '11? Windows users don't need an extra expenditure to use it.