> There's one thing I wonder about, why's there a difference between used+
avail. compared to disksize? Is this because of the fs?

--Most likely the -h flag is doing some rounding.
Code:
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/hde3              2298164    997984   1300180  44% /

Filesystem            Size  Used Avail Use% Mounted on
/dev/hde3             2.2G  975M  1.3G  44% /
--Using the second result, (1300*1024)+(975*1024) == 2329600, which is above the actual filesystem size. Even the actual (fdisk) size of the partition is only 2298240. If you need precice measurements, df w/o the -h flag gives the right results.

--FYI, I'm using reiserfs. If you're using ext2/ext3, a certain percentage of the fs is reserved for root (use tune2fs to check / modify == ' tune2fs -l /dev/blah ' to look ; ' tune2fs -m 1 /dev/blah ' to set reserved to 1%.)