So I'm not talking P-factor here, the airplane isn't yawing to the left, it's actually banking by itself, just slightly - say, 5 degrees or so - but enough that you have to constantly hold the aileron to the right a bit to compensate, which is rather annoying. I'm talking in single engine planes here, but it may do so in multi-engine planes as well, haven't tested.

Any ideas? I'm using the X52 if it makes any difference.

Sounds like a fuel imbalance..

Check if you set the fuel selector to "both" or just on one side. If you have selected a standard plane that doesn't allow to set the fuel selector on both, it will always be on left or right, even if you select another plane.

Brett Hendersons post is the most likely cause because right after take off in most light aircraft (In FSX) you wouldn't have burned enough fuel yet from one wing to cause such a drift. Make sure your default aircraft is balanced properly as the example below is not balanced. You see the icon in the aircraft symbol is slightly to the right so this plane will drift to the right. You can also see the fuel tanks have different weights. Set up your default as a balanced aircraft and save as default.



As Manuel said above, check your fuel selector switch before saving as default flight.

Edit: Check your payload in the same manner.