You're not just grabbing at the title bar, are you? You can grab anywhere on the window. I've never came across a situation where it is limited by anything but the boundaries of the mouse pointer itself.
I just tried it myself, if I grab at the title bar, it stops at the top panel on my Xfce desktop, but if i grab about in the center of the window, it goes right past it.
I do this quite often since I have a couple netbooks, many applications are made for at least 1024x768, since my screens are 1024x600 and 800x480, stuff gets cut off occasionally.