Our pals over at EngadgetHD took on one hell of a project and came out successful.
As much as we love the TiVo Series3, for $800 we would like a bigger hard drive than 250GB. Sure it's still almost 100GB more than most Cable Company DVRs, but we need more than 35 hours of HD goodness.A 500GB hard drive, a Linux distro and about a million steps later they were able to double the size of that TiVo hard drive.
Proceed at your own risk, though. We would hate to hear you bricking (or cinder blocking?) your $800 Series3 TiVo. If you are still too chicken to try this on your own, Weaknees sells pre-upgraded Series3 TiVos.
How-To: Upgrade you Series3 drive [EngadgetHD]