Update: (09/09/09) I have been told that I didn't really explain this well. Basically, the margin-top and margin-left should be 50% of the actual height/width of the div that you wish to centre. Sense?
This one goes out to maaa buoy Kripez. Just tryin' to rep da CSS. Whut?
Code snippet - vertical and horizontal centering with CSS on Snipplr
Result.
September 3, 2009 [17:36]