I just wanted to post how this problem was resolved, in case anyone else runs into it. I'm doing my Maemo development on a ~4 year old Sony Vaio (model pcg-5g3l). All was working well until I foolishly upgraded to Ubuntu 12.04 LTS. At that point Hildon widgets in my scratchbox Maemo 5 SDK stopped working properly. I re-installed an earlier version of Ubuntu, but that did not fix the problem. Anyway, I finally took reinob's advice (sorta), and set up a virtual machine to house the SDK. I used VirtualBox to build the virtual machine, and installed plain-vanilla Debian Linux as the guest OS. I then re-installed the Maemo 5 SDK within the guest OS VM. That fixed the problem; the Hildon widgets work properly within the guest OS even though they are still broken on the host OS. The Debian version I'm using as the guest OS is 6.0.5-i386.

Thanks again to the folks who made the helpful suggestions above.