iBooks runs a jailbreak check, and if it detects a jailbreak, fairplayd won't work. This used to be fixed using comex's iBooks fix, but AFAIK this has not yet been updated for 5.0

This is due to the fact that you probably have Activator by rpetrich installed. It causes a springboard crash when double clicking the home button because it tries to map an action to those home button clicks. Ryan Petrich, its developer, has said that he won't update his apps for 5.0 until the official version+jailbreak is out, so it looks like you're out of luck :( Hope this helps!

That would indeed be a very good idea. You wouldn't even need a Mac Pro that powerful, just one with at least 1gb ram for every user that will access it simultaneously. E.g. if there will be 10 users but only 5 that will use it simultaneously, 8gb ram will be more than enough. Hard drive space is up to you, depending on what you're doing with the accounts. Do remember, though, that screen sharing will be a little more sluggish than having direct access to the account especially when you're using a slow network. I'd get at least a 5ghz 270 Mbps time capsule/airport extreme in order to be able to use the accounts.
Do keep in mind that you will not be able to play music and the like because Screen Sharing doesn't support the transmission of sound or access to USB ports for that matter.
For everything else though, it would be perfect

NO UNLOCK jeez is it so hard to understand this blog? no unlock for that baseband, maybe a jailbreak.

yep, when a new jailbreak is released u can just use the current ultrasn0w to unlock.

No, if a jailbreak is released for the iPhone 4, you will be able to install ultrasn0w to unlock because you preserved your baseband. A difference in jailbreak tools does not affect ultrasn0w performance unless you are on a different baseband than the one that is supported: 1.59.00

if your iphone's model number is MB you don't have a 3GS but a 3G. 3GS's have a model number starting with MC

were you previously jailbroken (before 4.0) ?

he shut it down.

has a new bootrom, no 4.0 exploits for that just yet.