We bought with a Canon XT with the arrival of our second son. With the goal of pushing our learning, we started with the Sigma 30/1.4, Canon 50/1.4, Canon 85/1.8, and only used those for the first year. Turns out we ended up using the 50mm for probably 90+% of our shots -- as you know it's just a fabulous lens that was the most appropriate for most of our needs. The 85mm was too long, and we just rarely felt the need to go as wide as the 30. The 50 was just right for solo baby shots and close-up two shots of the boys. It also spoiled us with respect to image quality -- the pictures generally looked fantastic.

Last Christmas we jumped on the Canon 17-55/2.8 and are happy with it. It has proven nice for covering the wide angles we previously couldn't handle, as well as for being able to cover two fast-moving kids.

That said, I don't know that I would have preferred the zoom to the 50 during that first year for more than 10% of our shots, especially the baby shots. If you do use it, it will likely be for other content for now.

I would seriously consider the 580EXII recommended above. We don't have one yet, but when we've borrowed one it has done a fantastic job of lighting up the whole room instead of the on-camera deer-in-the-headlights flash. We tried to avoid using the flash unless absolutely required, but there are times when there just isn't enough light otherwise.

Best of luck with your shopping.

-m