Weird iPod touch behaviour

Proud owner of an iPod touch that I am for about a month now (more on that later), I noticed something odd today:
All of my downloaded apps (twitterific, facebook, rememberthemilk, ...) stopped working. They would sort-of-start but immediately return to the home screen before doing anything. The build in apps did all work nicely.

I googled for this, and found that I actually had to install an additional application from the app store, and lo and behold this did solve the problem. Whatever the reason...