When plugged in, the battery will charge until the smart charging circuitry and firmware say it is fully charged. At that point, charging stops. In order to keep the charging circuits from constantly cutting on and off as the battery slowly looses charge (all batteries loose charge, even when just sitting unused, they just do so very slowly), the system will not start charging again until the battery has depeleted by a fair bit (for most devices this will be somewhere between 4%-10% of the full charge). So, left plugged in, with a battery in good shape, your MBP will not begin charging again until the battery is say, somewhere down to 90%-95% of a full charge, which if just sitting idle would take at least a day or even two or three (depending on how old the battery is, and how much use it has had).
So no, you cannot overcharge it, no worries there.
P.S. my current MBP is a late 2008 model bought in 03/2009 - the battery has somewhere between 350-400 cycles on it (haven't actually checked it in awhile). It has spent probably 75+% of the time plugged in and other than when I'm away on vacation or something, it has been on 24/7 for the past 3 years (sleep timeout is 3hours, or I just close the lid when not using it). It does typically get unplugged and used in my lap for 20-30 minutes or more probably at least 2 or 3 times a week. Currently, I can comfortably work on battery power alone for up to nearly 3hours (using mail, safari, excel and likely word or a text editor, for example). So for a battery already well past 300 full cycles, it is doing just fine IMO. Oh, and I have probably calibrated the battery meter (ie. deep cycled it) only 6 times or so in those 3 years.
