This is my biggest and perhaps only gripe about the iPad. It doesn't have enough memory. My old iPhone 3g did the same thing, especially right before I got the iPhone 4 because the system software had gotten bloated in the last couple of years. Screen size doesn't help with this issue.
Apple either needs to spend some storage space on swap, or implement some kind of caching mechanism within mobile safari itself. The limited memory makes tabbed browsing incredibly frustrating, so much so that I use my iPhone 4 instead of the iPad most of the time now. I don't want to wait for a page load every time I switch tabs, especially over 3g.
The next gen iPad should ship with at least 1GB of ram, there is plenty of space in the case and ram is dirt cheap.
Edit: Looks like some people have already enabled paging by jailbreaking (http://www.ipodtouchfans.com/forums/showthread.php?t=288228). Apparently, iOS4 is going to enable paging across the board.
Keep in mind that space and price are not the only constraints when it comes to RAM. Quadrupling the RAM amounts will also quadruple the power requirements for RAM and will directly impact battery life, which is one of the iPad's greatest features.