Invisible App Store updates
It shouldn't display a blank screen.  It should either display "Loading..." or "All apps are up to date." or a list of apps that have an update available.

I've only seen this happen when I had a good internet connection when I opened the app store and started "Loading..." but lost the connection before it was done loading. I don't recall if it happens when switching from wifi to cellular or back during the loading stage, but I suspect it might.

I have hundreds of apps on my 3GS, though, and it can take quite awhile to go from "Loading..." to a list, or to the "up to date" notice.

It can also happen if you experience a temporary internet glitch during loading, such as if you are close to a microwave, using an 802.11b/g network, while the microwave is running.

It shouldn't happen when you know you've got a good internet connection and the connection state doesn't change during loading.  I suppose it could happen on a particularly busy network if significant numbers of packets are being dropped.

If it happens consistently, I'd try rebooting, then a system refresh, then taking it to apple in that order.

The app store is one thing that doesn't seem particularly robust in the face of network disruptions, so that's what I'd suspect first.  If you're experiencing frequent network disruptions in other apps as well, you might have an iOS or hardware problem, and it might be worth a visit the apple store anyway.