Exactly. The market for Rebel-class cameras, which is largely made up of former point & shoot users, expects pop-up flash. There's very little intersection between that market and people who would buy a 5D/1D class camera.

Serenus,

For brand-new cameras, the Canon EOS Rebel T1i, or the new Canon EOS 50D will fit a budget < $1500.

If you can swallow the modest difference in price, I'd say that that 50D is more fully featured, and would serve many advanced needs. The aforementioned Rebel T1i though is a terrific camera, just not as fully featured as the 50D. If you want more ultimate creativity in the longer run, the 50D is indicated, albeit it's hovering around ~$1K in price. Here are some links to help you decide (perhaps copied in duplicate from other posts):
www.dpreview.com/previews/canoneos500d/
www.dpreview.com/reviews/canoneos50d/

A very worthwhile feature on both the T1i and the 50D is the 3.0" 920K pixel LCD screen. Even my professional Canon EOS 1D Mark III camera does not even have this LCD capability, kinda annoying really. The extra detail that is visible this way is going to be immensely helpful for critical previewing. Such as for product or print work, where little details matter. Now you could work tethered to a computer, but that's not always useful or easy, for example, for a location shoot. The very fine display in the LCD will help a lot when having the laptop tied to the camera is not useful or possible. Another example I can readily think of is using Live Preview for astrophotography, where you can watch the Moon and stars directly in the LCD, when the camera is tied to a telescope. This view with the (digital) zoom engaged Immensely helpful for tack-sharp focusing with the telecope's focuser. On my 1D Mark III, with Live View you can literally see the wavering atmosphere against the fleeting lunar landscape; with the T1i/50D, the detail would be enormously better while waiting for "still air" to take the best shot.

To the extent you are willing to climb > $1000, you'll will then have budget for non-professional or pro-sumer lenses and/or a Speedlite accessory. A Speedlite, plus a flash diffuser will enable much better people photography than the built-in flash can offer.

What lenses that may be best for the camera depends on what you want to do. However, for modest low-light, there is always the ~$80 "Nifty Fifty" 50mm f/1.8 lens. Also, medium speed zooms (e.g. f/3.5-4.5) may more tenable now, because on cameras like the 50D, the highest ISO with good noise quality is ISO 3200; however, incredibly fast ISO 6400 and ISO 12800 are even available on this camera, which allows the use of slower zoom lenses in places that heretofore were not tenable, even with higher noise. This situation might not be so bad photographing (say) a fire arts performance in the near dark for a Burning Man type of gig.

Hope you have great fun and much creative work, with whatever DSLR you decide to obtain.

Blessings and Light,
M