So I took my (1st Gen) Time Capsule apart and switched the old 500GB drive for a 3TB drive - which worked in principle. I was impressed that the old thing could handle a 3TB drive ok. The problem that I increasing encountered was that Apple's File Sharing (AFP) server kept crashing - which basically meant my Time Machine stopped working and I had to go and manually switch it on and off every day.
To fix this issue I took the disk out of the Time Capsule and put it into an externally powered USB 3TB capable enclosure so that Time Capsule finally worked reliably again (and no longer crashed). The reason this worked was because the crashing was down to overloading of the power supply (which led to the crashes) in the Time Capsule itself - by moving the disk outside meant that the power to main board of the Time Capsule was now more stable, plus it wasn't getting over heated (which is another problem that besets such small embedded systems).
But along the way when I took the hard drive out of the Time Capsule and tried to look at it on my computer I only found two rather oddly named partitions - APswap APconfig. After searching on the net it seemed there should have been a third partition named Data but it was not showing up?! I was very puzzled as it showed up when inside my Time Capsule. After a lot of faffing about I finally discovered that problem was down to a hardware issue - all the harddrive enclosures I had tried with my drive were not actually capable of reading a 3TB drive - though they were all happy reading a 2TB drive (but quite few enclosures can only read 1Tb or less - so check this if you're having this issue). So I finally managed to find a 3TB capable enclosure and it reads the disk fine and mounts the Data partition (and may be used for external connection of the disk to my Time Capsule so it works as I explained above).
I did also try to mount the disk from Linux but the default filesystem used by the Time Capsule is Apple's HFS+ with journaling enabled used to be not so straightforward to mount. It seems that Linux now mounts hfsplus disks with journaling switched on without a problem (though I'm not clear whether what happens to the jornaling?). On older versions it's also possible to force mount the drive using Linux's hfsplus filesystem driver. It seems that under Lion it's no longer possible to switch off journaling on HFS+ using Disk Utility which is a pain - though I understand that it can be switched off using the diskutil command.
One final thing I tried was set up a Linux box as Time Machine capable network drive. One thing I learned along the way was that Linux's HFS support (i.e. Apple formatted disks) is not rock solid and it would crash when running the backups so I had to change the disk format of my backup drive to use Linux's ext3 filesystem instead (then I could use the ext2/3 fuse filesystem to mount it on OSX). This basically seemed to work on one of my machines, but unfortunately it seemed less reliable on another (It lead to this error message every so often: "Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you." - which makes it useless) so I've gone back to using the old Time Capsule with an USB external disk (having removed the internal one which means it no longer crashes).