Choosing my next Canon lens

Need lens-buying advice from Canon photography nerds.

The Mrs. and Me are starting to really dig taking photos again – she with a Rebel XT and me with a 30D. We currently share joint custody of 1) the XT’s standard 18-55mm zoom (blech) and 2) the glorious and much-loved 50mm f1.4 (yay!).

So, let’s say, hypothetically, a fella wanted to pick up a new Canon lens to add to these two cameras’ modest arsenal. If we wanted a fast, flexible lens (that would mostly get used for baby picture-taking), what would you guys recommend? Possibly a higher-end, faster zoom?