Let's put it this way...from experience coming from a S90 to 550D.... regardless whether you get the 50D or the 550D.
You'll be totally blown away by the performance even with the bundled lens.

Then after that, you'll expect more and more from it until you start knowing what you want and start spending more than you can afford on lenses
My 2 cents is that:

If you're looking for best optical performance for the buck and you know for sure that you won't be spending $$$ on this later.

- 18-55mm IS
+ 55-250mm IS
+ 50mm f1.8

This is the best bang for buck combo which allows you to cover the entire FL range and a nice portrait/low light capability.

If deep down inside, you know that this is just touching the surface and you have no qualms about "investing" in quality glasses later.

- 18-135mm just to get started. Zoom zoom here and there. After getting the right optics for the right purpose and all... you'll still have a decent travel zoom for those days that you just wanna jalan jalan without being over burdened by tonnes of lenses.

Hope this helps.

if u r not in a rush, wait for next upgrade for D90 to come out. the price of D90 will certainly drop.

Hmm...Still undecided.
Now my friend offer to sell me her 6 months old D90.
I would consider if the price is reasonable.

I shoot using my friend 550D today. I feel the image quality is nice!!!
Much more better than NEX5 and EP1.

Btw, I will try my friend D90 tomorrow...