You may be running out of disk space fast if you have small SSD start partition on OS X Lion if you leave MobileBackups enabled.
If you open Terminal and run the command "df -h" you might see the (otherwise hidden) directory .MobileBackups using up a lot of space.

This feature is described in the Ars Technica OS X Lion review for example. It is there so your laptop will run backups even when you're on the move with no backup disk attached.

To disable it, run the command "tmutil disablelocal" in Terminal.
To enable it, use "sudo tmutil enablelocal"

macosxhints.com now have an easy solution to enable backup on mounted network drives with Time Machine: type the following in your Terminal:

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

and Time Machine will let you select mounted network drives (under Set up Time Machine / Change Disk). I have no idea how reliable it is to use Time Machine with network storage and what kind of format is best. I still use a Firewire disk. Another issue is, that Time Machine will eventually fill all available disk space, no matter how many TB you have (more on this issue discussed here).