I have only had my HTC desire for over 1 month and managed to full my SD card. (I previously have also run out of onboard memory for apps - but that issue should be resolved by the Froyo release of android - where I will be able to store applications on the SD card).

The full SD card problem - is different in that I know I have K9 mail installed and loads of other apps. I also thought the problem might have been caused by accidentally recording a very long video. I spent a lot of time looking for a app in the market place - but had no luck. Eventually I attached the phone to my macbook (using the USB cable) and pointed Jdisk report to my /Volumes/No Name on my mac to get a pie chart of my SD cards disk usage. It was doggcatcher (a podcast downloader) that is using all the disk space.