Yes, if it's big enough (see question #1). Time Machine keeps track of which computer is which (by a hardware identifier, not computer name), so it will always keep its backups for each Mac separate from each other.
It is advisable, though, to partition an external disk that will be connected to your Macs into one volume for each Mac (see question #5 or #6 for instructions). You can't partition a Time Capsule's disk, but each Mac will have a separate sparse bundle on a Time Capsule, so it's not as big an issue.
There are three reasons for this, for a drive connected directly to your Mac:
Time Machine will, eventually, fill all the space available to it before it begins deleting old backups. When multiple Macs are "competing" for the same backup space, there might be room for several months of backups for one, but only a few weeks for another. This is especially likely to happen if you start backing-up a new Mac to a drive that already has a lot of backups from another Mac. Time Machine on the new Mac will not delete backups from the other Mac(s), so when it needs space for new backups, it will delete the oldest backups from the new Mac instead.
If you ever want or have to delete all the old backups for one Mac and start over, you can just erase the partition via Disk Utility; if there are other Mac's backups there, they would be erased also. If you replace one of the Macs, Time Machine on the new one won't delete the backups from the old one. And sometimes it's advisable after certain problems.
You can't copy one Mac's backups to a different, larger drive or network destination.
If you're backing-up to a USB disk connected to a Time Capsule or Airport Extreme, you can use separate partitions if desired, but it's less of an issue, since the backups for each Mac will be in separate sparse bundles. Those can be deleted via the Finder if necessary.
If you're backing-up to a shared disk connected to another Mac on the same local network, there should be a separate partition for the Mac that the disk is connected to directly; the other Macs can share a partition or have their own, as you desire, since these will each be in separate sparse bundles.
It is possible to delete individual backups via Time Machine's "Star Wars" display, but it's one-at-a-time, so rather tedious. See question #12. (Don't be tempted to just delete them via the Finder -- all sorts of things may go very wrong.)
