The 6-cell battery is flush with the system.
On my E1505, the 9-cell is also flush. But I think with a 14.1", it gets out of the system.
Personnally, I would not choose the D520. It doesn't have a widescreen display ... It is a question of taste, but I prefer a widescreen display ( Windows desktop is bigger, highest resolution ). If you want a 14.1" laptop, then, take a look to the D620. But if you want a traditional square display, then, the D520 could be a very good choice. Dell quality is very nice ( especially in the Latitude line ).
I am looking at buying a Dell Latitude D520 and would like to have the 6-cell battery, but only if it is flush with the system. Some laptops have a high capacity battery that sticks out of the system, making it more bulky. If that is the case, this battery is no longer an option.