Though I stand by my statement that the argument over shape is irrelevant, I find Leolaia's comment on Christ carrying his "cross", "stake", etc., very interesting.
Logic would tell you that it is unlikely that Jesus, even though a perfect man, would be able to carry a "stake" large enough to be crucified on to his execution site. I would suggest that even a cross-member would be very heavy and unwieldy for a man to carry any distance...especially a man who had been tortured and beaten!
Throughout history, specified execution sites had "re-usable" instruments of death in place, ie: hangman gallows, firing squad walls, electric chairs, gas chambers, etc. Would it not be reasonable to believe that the Romans had set areas for crucifixion? Would it be reasonable to think that they had the stakes already in place and then the person being crucified could be placed on the stake via a cross-member?
Again, I don't know why the shape of the instrument of death is so critical, but I think there are PLENTY of reasons to believe that an upright stake was not a very feasible method of crucifixion...