Hey,
a short while back I went looking for a TextMate plugin for ack but the advertised link was dead so I based one off GrepInProject++ (instead of doing something sensible like contact the plugin's author about the dead link...).
Anyhow, my own take on "Ack in project" can be found at:
http://github.com/protocool/ack-tmbundle/tree/master
It does nifty stuff like remember your last 10 searches, allows you to choose word, literal or case insensitive search as well as configure options like whether you want to show context lines, follow symlinks and ignore .ackrc (you can define a .ackrc in your project directory too).
I've still got a few things I'd like to improve with it but it's in daily use (on Tiger, will test Leopard shortly).
Regards, Trevor