remote backups

Apple posted this at http://docs.info.apple.com/article.html?artnum=306652

Time Machine backs up to either an external disk (such as a USB 2.0 or FireWire-based third-party > drive), a secondary internal disk or partition (if your computer supports additional internal drives), or > an available Mac OS X Server version 10.5 volume. Although you can still burn personal discs in
Leopard whenever you want, Time Machine itself does not back up to burnable DVDs or CDs. One
purpose of Time Machine is to protect your data in the case of issues with your Mac OS X Leopard
volume, so it only backs up to a separate volume.

If you plug your external drive into your main desktop mac, when you network your other computer(s) to it, they will automatically detect the external drive. Time Machine will then autodetect the drive and start backing up to it wirelessly. TM should create a separate sparse bundle for all the different computers networked. You can also access TM remotely if your desktop is set for remote access and the ext drive is plugged in. This way you don't have to unplug your ext drive from the desktop if that is your central computer. I only discovered this by accident when I was attempting to network my MacBook with my eMac and my external drive just happened to plugged in to the eMac at the time. Previously, like yourself, I was plugging and unplugging the drive between computers, which really annoyed me after a while.
I hope this helps

But TM will make a new backup from scratch in the case you connect to the backup drive over the network, right? Because this is what is happening right now – even though I already connected the hard drive locally to my PowerBook, it doesn't seem to recognise the existing backup when accessed over the network ... Slightly annoying, if you ask me ...

The problem is that TM backs up two different ways, depending on whether the disk is directly attached or on a remote Mac. If local, TM creates a folder and places the dated backups in that. If the disk is on a remote Mac, then a sparse bundle is created. If you try to use the same disk in both ways, you will get a full backup once for each case. After that, the backups should be incremental.