There are a few things that you can add to your Canon setup that will pay-back over and over again.

1) An external flash, cord and flash holder. At least get your hands on the 580 EX or 580 EX Mk II. Bounce the flash off of ceilings or put a reflector on it. Makes indoor night / or low light shots so much better than the TERRIBLE built in flash. The world would be a better place if the 30D and Rebel's simply did not have a flash.

That said, there are a few lenses to consider as your next lens.

You are on the right track with your 50/1.4. That is a fantastic lens , and on the 1.6x crop factor Rebel and 30D, it is the portrait lens of choice really. I used to shoot with an EF85/1.8 USM as my portrait lens of choice until I went digital. The conversion crop leaves the 50/1.4 as the only serious replacement for the 85. Both lenses are standard construction and produce amazing results. I routinely prefer images from the 50/1.4 to any of my L zooms.

With that out of the way, a versatile zoom would be the 28-70/2.8L (or whatever today's equivalent is) or the ultra-wide L series zoom (17-40/2.8L). I would suggest that your money is better spent on the 28-70/2.8L since in the medium term i would expect the re-introduction of full-frame digital bodies that are affordable (around the same price as a 30D), and you will not want to have invested in EF-S lenses that prevent you from moving to the full-frame bodies. It is also largely for this reason that I would avoid the ultra-wide zoom, unless landscape and panoramas are your passion. For shooting people, you want to have the 28-200mm range well covered and the 28-70/2.8L zoom is a great lens in this range. It will remain just as useful with the full fram bodies.

Note that I realize you already have a 50/1.4 and that the 28-70/2.8 overlaps. HOWEVER. if you are travelling, the 28-70/2.8 is a nice one-stop lens and the 50/1.4 is really more about low-light performance and amazing Bokeh.

Enough said - I'd run out and get the 28-70/2.8L. If money is a concern, lots of people like the EF-S 18-80 IS USM (??) lens, but I have zero experience with it. Once you shoot primes and L lenses, it is VERY hard to go back to the prosumer zoom lenses.

I have a selection of image at http://flickr.com/photos/muonzoo/ mostly shot with a 70-200/2.8L and the 50/1.4. My next lens WILL be the 28-70/2.8L for the convenience ( I have 3 primes in that range ).

Best of luck

Alan