If you're running MCPC+, grass growth is fixed in the latest 1.4.7 recommended build:
http://mcportcentral.co.za/wiki/index.php?title=Mods_and_Plugins_for_1.4.7 - it was an issue in a couple dev builds after 244.
Also by default the tick rate is limited for performance reasons (tuned for large servers). The vanilla rate can be restored by setting aggregate-ticks: 0 in bukkit.yml.