I've got lots of Apple IDs. I've got an iTunes account with all my apps and music on it, we have family MobileMe accounts for email... what does the advent of iCloud mean for me?
We're not sure how Apple is going to handle multiple accounts. However, the iOS 5 beta features an iCloud settings item as well as a separate Store settings item. The iCloud item handles the syncing of mail, contacts, calendars, reminders, bookmarks, notes, Photo Stream, and Find My iPad, as well as storage and backup. The Store setting seems to cover music and apps. So it's possible you'll be able to use one Apple ID for media purchases and syncing, and a separate ID for all the other data. In this way, if you have a single family account for media purchases, you can keep everyone on that while using a separate account for other data.
Will I be able to merge my iTunes and MobileMe accounts into a single account? Or change my Apple ID and retain my old apps and media?
Apple hasn't said anything about how management of Apple IDs will be handled. Given their increasing importance, it would certainly be nice if Apple allowed users to change, merge, and even split accounts.
If I have an expired me.com email address from a MobileMe trial, or a mac.com AIM account, will I be able to claim them when iCloud is released?
It seems likely that if you signed up at some point for a me.com or mac.com address, you may be able to re-activate that user name once iCloud launches. We won't know for sure until that day gets closer.
Can I play music directly from iCloud, or do I have to first download them to a device?
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 email 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.