Share memory access on SD card between a PC and an Android App
There is apparently an app in the Market that does this.  I do not know the name.  It probably requires root.

However, doing so is a terrible idea.  Two operating systems should NEVER access the same disk at the same time, because they do not know about each other's writes.  If you do this you will lose data, possibly everything on your card.  There is no advantage to this anyways unless you are doing something extremely bizarre.