Wow, I didn't realize so many people didn't even bother straining. Does leaving the hops in affect the flavor at all? I would think it would result in a more bitter beer, since the hops has more time to impart its hoppy goodness.
marubozo is right, the bittering comes from the boil. leaving hops in won't make the IBU's different. Alpha acids aren't isomerized below boiling temps.
However, it isn't correct to assume that leaving the hops in won't affect flavour at all. By this point, we all know enough about brewing's many nuances to know that virtually every choice we make can affect flavour. I will say that doing so won't ruin your beer, that's certain.
So while it won't necessarily make better beer, or worse beer, leaving hops in the beer when fermenting exposes your beer to hops differently than if you removed them.
Hops you add at flameout (0 mins) don't stop contributing flavour when you pitch the yeast. They will keep doing so for many days, some falvours can be good, some can be bad. So while not 'dry hopped' this is very similar, the major difference being that the hops were initially put in the wort at higher temps, near boiling. So all else being equal you may get more flavour from leaving them in the wort than removing them.
Also, hops have anti-bacterial properties. We all agree on that. Well, hops can slow yeast the same way they slow bacteria. Again, the effect isn't huge, and it would take a lot of hops to slow all 200-300 billion yeast cells we're pitching. Depending on how many hops and how much yeast you pitched the yeast will still ferment well, but attenuation could be affected.
I'm just listing this stuff to outline some issues that are worth knowing. These aren't major consequences.