I'm bringing this back!

The premier PS3 exclusive game vs the premier Xbox360 exclusive game

Right now, GT5 has sold 7.36 million, and Halo Reach has sold 8.89 million.  Gran Turismo 5 is still selling somewhat well and outselling Halo Reach every week.  GT5 can have another big boost in sales during the next christmas season and some minor sale boosts with few more DLCs.

You'd do better if you included charts and what not

Don't think so. The XL edition of the game already came out so don't see any big boost happening.
Halo Reach will get Christmas boost as well you know.

Perhaps, but it's not like Halo 4 isn't going to outsell both of them.

It depends whether GT5 is going to be bundled next christmas, or maybe with a new slim revision.

it seems pretty clear that, short of massive downward adjustments for halo reach or sony bundling GT5 with every single ps3 next christmas, GT5 will not pass it. It could close the gap a bit. Digital Copies of Halo Reach have been pretty heavily bundled over the last holidays and now with the xbox live deal, so plenty of new 360 owners are getting their obligatory halo game for free which no doubt has hurt sales recently and its not gaining anything "vgchartzwise" from these bundles since they are digital. Halo 4 will further damage reach's legs when it releases these holidays, but next gen consoles are probably coming in 2013, and GT6 won't take as long to come out as GT5 did. 

The OP tailors his argument to make it fair so Gran Turismo 5 has a chance of pulling it off.

So it is Gran Turismo 5 versus Halo Reach instead of Halo 3. 

1.5 millin gap is still too much to pull back for game that has been out for 18 months and the generation ending within two years.

Answer: no.

Extra answer: The comparison is invalid due to the inability to count bundled and digital sales, so an extra: no.

This. Since it's a prequel I don't think too many expected Halo: Reach to outsell Halo 3.

(One could argue though that Call of Duty stealing Halo's marketshare is the main reason to Reach's sales. And then there's the digital sales...)

Add another 2 million sales for Halo: Reach due to bundles VGC did not count and you might get your answer.