If you are running Mac OS X Leopard, you need to do the following.

Open up a command terminal.
Run the command "dscacheutil -flushcache"
If you are running Mac OS X 10.5.1 or below, you need to do the following.

Open a command terminal.
Run the command "lookupd -flushcache"

then run
dscacheutil -cachedump

clear browser cache.. and BOOM!

instant DNS update no wait. works for me all the time. MAC FTW!