If all goes well, and Phobos-Grunt is successfully launched
towards Mars in October, 2009, it may [again depending on success or not]
return to Earth with BOTH Phobos and martian surface/subsurface samples.

Understand that when early Mars was battered by asteroids, trillions
of tons of martian material was ejected into deep space. Surely
upto a few hundred tons of martian material may have been deposited
on the surface of Phobos as a result.

In other words?
The Phobos-Grunt spacecraft won't be able to discriminate between martian
and Phobos material...material that may be mingled on that moon's surface.