If the computers have WiFi toMACsh, they should be able to connect to the TC and backup (of course, at that point, they are now part of your wifi network and are no longer "not networked" - they are just not wired networked)
To Michael415: yes, you can use a Drobo as a TM backup device - I don't know if it will work on a airport extreme, but you do have to take some precautions.
For those that don't know what a Drobo is, it's a special raid device that allows you to dynamically grow your storage, including yanking and replacing existing drives with larger ones. There is a trade off to this - for it to work, you have to format the drive for the overall size you'd *like* it to be - ie: let's say you have 2x 500g drives and you put them in the Drobo - the drobo will technically raid them in a mirror giving you ~500g of
real useable storage. BUT when you format the array (which you need to do thru their software) you setup what the max size of the array
one day[/b ] can be, so you can set it up to be 8TB. The OS will see a 8TB drive w/ ~8TB available, but in reality since you only have those 2x 500g drives mirrored, you only have 500g available. The only way to know for sure the available space is to either: use the drobo software which will always tell you your real space available, or look at the pretty blue lights on the front and guestimate how much space is used. This is why you have to take precautions - since the OS will think the drobo storage is larger then it really is (potentially) it won't know when the drive is really out of space and keep attempting to write to it. In the past people have created sparesbundles to setup a max storage size for the TM backups.
But, here's some info right at data robotics: What do I need to keep in mind when using my Drobo storage device with Time Machine, AirPort Extreme, and/or Time Capsule?
If the network drive is recognized via SMB take a look at this: 10.5: Store Time Machine backups on an AFP NAS - Mac OS X Hints
I do own a DRobo, but honestly, I don't use it for my time machine, I use it solely for my video/music bank for my network at home.