Not sure if disassembling the PS3 is the only way to get it quiet agani, but I did try cleaning with a rather powerful vacuum cleaner... sadly to no avail.
I suspect, it must be pretty dusty in side which is causing it make more noise (heat up more quick) and the only way I know of to get to the fan, is to disassemble it... I'd much rather just buy a slim PS3 though.
If you have a non-BC PS3 you may use the fan-test to blow away some dust.
http://www.gamegrep.com/news/8618-the_playstation_3_can_clean_itselfs_of_dust/
http://www.1up.com/boards/posts/list/36098.page