You need the growlnotify utility that comes in Growl Extras on the Growl installer disk image.
Create the following AppleScript and save it. Then create a match-all rule in Mail.app (Mail » Preferences » Rules » Add Rule) to run the script as the performed action. Don't apply it to the existing messages, just new ones.
using terms from application "Mail" on perform mail action with messages theMessages for rule theRule tell application "Mail" repeat with m in theMessages set sub to m's subject set sndr to m's sender set msg to sndr & " " & sub -- display dialog msg do shell script "/usr/local/bin/growlnotify -I /Applications/Mail.app -m " & quoted form of msg end repeat end tell end perform mail action with messages end using terms from
[crarko adds: I tested this, and it works as described. There was a previous hint about making GrowlMail work with the current version of the OS, but this is a pretty good alternative too. When I created my test rule for this I just used 'if any recipient contains .com' as the matching criterion, since that matched all my accounts. But you can obviously customize the mail rule as much as you want.]