If you've been around here for awhile or seen any of my presentations, you've heard me speak a little on the concept know as flow. When building your interface, the absolute ideal that you should be striving for is helping your users achieve a flow state, that frame of mind in which everything else seems to fall away and you become completely immersed in your task, distractions are eliminated and time flys ("I've been doing this for 5 hours??? It seems like 5 minutes!"). There's a new article on A List Apart that describes this much more elegantly than I can, so you may want to head on over there and check it out.
A List Apart: Designing For Flow
Have a great weekend everyone! Only 8 more days until Lotusphere!!! :-)