Let me say right up front that I don't own an entry level dslr so a direct comparision is not possible here. That said, I did own a 10D 6MP camera so i'm perhaps not entirely 'talking through my hat'.
The FZ50 provides 35 - 420 mm. in a single relatively compact, quite light unit, with image stabilization and noise issues for $650 Canadian.
A Rebel XTI is a bit bulkier in body and were you to put on one of those third party 23-300 mm. lenses and allow for the 1.6 multiplication factor, you'd end up with a package that isn't a whole lot bigger and perhaps 50% heavier than the FZ50. Price would be $900+$400 for the lens or about double the price. There's no image stabilization but you can quite easily increase the ei. by a factor of 3 stops to get the equivalent effect (and stop moving subjects better). That would increase base ei from 100 to 800 and noise is very approximately equivalent to the FZ50 at 100.
Thus the two packages are really rather similar.
The DSLR gives you the option of using a lower ei. than 800 when it isn't needed for truly superb results. On the other hand the FZ50 has a superb lens which walks all over even some of the Canon zooms with superb central sharpness and excellent edge sharpness, instead of adequate centre sharpness and mushy edges of a 28-300.
If you allowed yourself to carry multiple lenses and buy good glass, there would be no comparison, but that wasn't the question was it. The price now goes up to $3000 for camera and at least two lenses and it now weighs something like 5X as much as the FZ50 and requires a 'real' camera bag.
There are other factors which may sway your decision one way or the other
- tilting lcd on the FZ50
- no lens changing and therefore dirty sensors with the FZ50, potentially making it a better choice for iffy environments
- greater depth of field with the FZ50 - can be a blessing for landscape photographers and a problem for portraitists
- dramatically faster focusing with Canon lenses on a Rebel XTI or Nikon D40X - a make or break issue for someone photographing sports (or whales).
- raw shooting with DSLR's is a lot faster
- jpegs with the FZ50 are remarkably smooth but are VERY painterly at 100%. This affects large prints but generally not small ones (eg. 8X10).
- the FZ50 can be very unobrusive, with waist level viewing and no sound image capture. That might be important to some.
- some people can't stand electronic view finders - raised on the Olympus 2100 then Sony 707, it doesn't really bother me but there's no doubt an optical viewfinder is better - still the FZ50's is pretty good.
- the noise on the FZ50, present to some degree even at ei. 100, is quite film like and particularly in black and white, can look very like 35 mm. tri-x. OK, that's not at 400 ei. but with the depth of field afforded by the small sensor and the image stabilization - photographs of tri-x 35 mm. quality are fully to be expected, and that's no bad thing.
So, who should by an FZ50? As a fun, walk around camera suitable for modest size prints (13X19 with a decent border)it has a lot to recommend it. As a landscape camera backup, it probably makes more sense to park one of the 10 MP dslr's on a tripod and use good lenses. For sports and action, there's no question that a dslr is the way to go. Did I do the right thing with the FZ50 - so far I think so - it's been fun using it.