I can attest that it sucks up memory for me too. I'm on a Powerbook G4 1.67Ghz machine with 1.5 GB of RAM installed. Here's a screenshot of my Activity Monitor with OW highlighted.
I've had OW open all day, but the whole computer is shutdown each night. I never have more than 8 tabs open at once.
I'm more than happy to send any logs or about any other kind of information OmniGroup might find useful in identifying such a high memory usage.