I came upon something relatively recently (within the past couple years), in one of the 'incredible cross-sections' Star Wars books. In the listing of statistics for various Episode I-III spacecraft, the speed in the atmosphere was dependent upon whether or not the craft had its shields up or not. And the difference between shields up speed and shields down speed was quite dramatic (to the tune of several thousand KPH).
This is actually kind of an interesting new development that has never been talked about before- certainly not in the Star Wars roleplaying game. What it seems to suggest is that starship shields can be 'shaped' to give a craft a dramatically better aerodynamic profile. On the one hand, this seems to make a lot of sense. Afterall, shields obviously have an effect on matter and energy- so why wouldn't they work against atmospheric particles. It would certainly help explain why the Millennium falcon (not exactly the most streamlined of ships with that huge radar dish), could soar through the atmosphere of Hoth and Bespin without any trouble at all. In fact, the 'shields as aerodynamics' could help explain away a LOT of things we see in the movies that would otherwise be some major physics problems (of course, it still doesn't explain why Lambda-Class shuttles have such huge wings...).
On the other hand, it kind-of makes you wonder how MUCH effect this aerodynamic shielding would have. If it is as much as the books show, then any non-shielded aircraft would be next to useless in trying to fight a shielded one (at least speed wise). It also makes you wonder why any fightercraft would NOT have shields. The TIE fighter, for instance, would likely be crap in an atmosphere, 'realistically' speaking, what with those huge panels. Any kind of crosswind would be a bitch to handle, especially considering they don't seem to have any control surfaces on them.
That's why I'm inclined to allow shields to have SOME aerodynamic effect on craft, but not as drastic as some of the books suggest. In fact, I'm just good knowing that shields can help to 'gloss over' why brick-like Starships can fly in atmosphere at all.