There are some things a full-auto compact can't do that makes them giveaways even at web size, such as long exposure. (note: I'm referring to compacts as those without M,A,S,P; kay fine actually they all have Program mode but... full auto) At full size, compacts are probably going to give significantly noisier images in bad light due to their small sensors. There is probably more ways to tell...
That said, it doesn't really matter if you can tell if a photo's been taken by a dslr or compact. I've seen many photos taken by DSLRs that look, well, bad, and many photos that have been taken by compacts that look pretty darn good.
First of all, PnS are meant for very general shoots... that means normal lighting conditions, non extreme conditions (really high ISOs, really long exposures, etc, etc, etc)... If you drop the DSLR to do such easy images, you're bound to get the same similar results. It is like forcing the Ferrari to go at the speed of Honda City.... wow, can't see the difference in speed right?
Hence, this is a odd (I'm bordering on calling it "stupid") comparison if you compare photos taken under normal lighting conditions (call that easy level)...
What you should really compare is... indoor dim lighting conditions, ultra high speed shots (freeze motion yet noise level minimal and still pin-sharp details), portraitures shot at f/1.2 & f/1.4 apertures (AT night)... HANDHELD! Try comparing the shots taken by PnS and DSLR... I double dare you to show that there aren't really any differences at all!
Sure, drive a Ferrari at Honda City's normal speed you're bound to experience the latter's performance... but don't forget that you are still able to leap from 0-100 km/h at 3-4 seconds flat... Try doing that with the Honda City!... Good day... :bsmilie:
Edit: You might as well ask: Ferrari versus Honda City, which one is the slowest, most comfortable, and MOST fuel efficient!? Honda City wins!!! YEAH!!!! lols
Double Edit: some of us use DSLRs for normal lighting conditions that already can be handled well by PnS... Well, I don't - love to push towards the limits of what my lenses and DSLRs can deliver..