You are not supposed to "wiggle" the pin to see it's input; it should be enough to simply *leave* it in its reset state, which is one.

This really sounds like if you have some sort of pulldown connected to the pin. If the pin is pulled low externally by a switch (practically zero impedance), then after the switch releases, further pulled low by some "weak" pulldown (in the order of few tens of kΩ), it remains low - the weak internal pullup can't source more than a couple tens of uA. However, if you then write to the port 0 then 1, for a brief moment after writing the 1, a strong internal pullup is switched on, sourcing several mA, overcoming the pulldown; and once the pin is in one, a medium internal pullup keeps the input in 1 (sourcing hundreds of uA).

This is all "biblical" stuff, but can catch the unaware easily. See also Kai Klaas's excellent explanation here: http://www.8052.com/forum/read.phtml?id=121603

JW