UPDATE: You can grab a beta version of 64-bit Growl 1.2 from the Growl beta site.
GrowlMail is broken on Mac OS X Snow Leopard (10.6). The dev team for Growl has a fix in the works, but I use Prowl and like to have notifications go to my iPhone when I’m not at my computer. So in the meantime, here is my solution.

I was originally inspired by James Higgs.
You first want to create a new AppleScript using AppleScript Editor and save it somewhere logical. I used /Library/Scripts/Mail Scripts/Rule Actions/Growl.scpt Below is the code:
-- Growl Alerts in Mail
-- Hunter Ford [http://www.cupcakewithsprinkles.com]
-- This script arises from the lack of any Growl Support in Mac OS X Snow Leopard (10.6)
-- Code inspired by and adapted from James Higgs [http://blog.jameshiggs.com/2009/08/28/growlmail-on-snow-leopard-a-temporary-fix/] as well as those mentioned.
tell application "GrowlHelperApp"
-- Make a list of all the notification types
-- that this script will ever send:
set the allNotificationsList to {"New Email"}
-- Make a list of the notifications
-- that will be enabled by default.
-- Those not enabled by default can be enabled later
-- in the 'Applications' tab of the growl prefpane.
set the enabledNotificationsList to {"New Email"}
-- Register our script with growl.
-- You can optionally (as here) set a default icon
-- for this script's notifications.
register as application "Mail" all notifications allNotificationsList default notifications enabledNotificationsList icon of application "Mail"
end tell
-- Mail Rule Trigger
--
-- Source: Benjamin S. Waldie [http://www.mactech.com/articles/mactech/Vol.21/21.09/ScriptingMail/index.html]
using terms from application "Mail"
on perform mail action with messages theSelectedMessages for rule theRule
repeat with thisMessage in theSelectedMessages
-- Process the current message
-- Grab the subject and sender of the message
set growlSubject to subject of thisMessage
set growlSender to my ExtractName(sender of thisMessage)
-- Use the first 100 characters of a message
set growlMessage to (content of thisMessage)
set growlLength to (length of growlMessage)
if growlLength > 100 then
set growlMessage to (characters 1 through 100 of growlMessage) & "…"
end if
set growlMessage to growlSubject & "
" & growlMessage
-- Send a Notification
tell application "GrowlHelperApp"
notify with name "New Email" title growlSender description growlMessage application name "Mail"
end tell
end repeat
end perform mail action with messages
end using terms from
-- *ExtractName*
--
-- gathers the name portion from the "From: " line
--
-- Source: robJ [http://forums.macosxhints.com/archive/index.php/t-19954.html]
to ExtractName(sender_)
if sender_ begins with "<" then
return text 2 thru -2 of sender_
else
set oldTIDs to text item delimiters
try
set text item delimiters to "<"
set name_ to first text item of sender_
set text item delimiters to oldTIDs
on error
set text item delimiters to oldTIDs
end try
return name_
end if
end ExtractName
Run this first in the AppleScript Editor. As this is what will “register” the notification with Growl. Click the green “play” button that says “Run”.

Then you want to create a new rule in mail that will run this script on every message that comes in.

Make sure to select “Don’t Apply” that way you don’t flood your screen with tons of Growl notifications.

