Actually, I think it's not so much that the keyboard connects quickly that Apple created a special system for pairing its Bluetooth keyboards at firmware boot -- I think it actually then re-connects via the actual normal BT stack later in the booting process. Because it's not just a matter of quick -- you can not only do things like hold option at boot, but you can boot into Open Firmware (e.g. on the PPC's) and edit things using the wireless keyboard without OS X even running at all, something that is not generally true for BT keyboards on Windows PCs.
Whichever way, though, they do an extremely nice job with it.
On my iMac, I have never, ever needed to plug a wired kb or mouse into it to be able to get it to do anything. In contrast I have had to disaster recovery on my EeeBox in WinXP with a wired keyboard because I couldn't do low level stuff with the BT keyboard it normally uses.
Very nice -- one of those little things I think Apple implemented extremely well.