Not sure if this is a bug or something specific to my environment, but some widgets that display just fine in my 1.4.2 instance don't show at all in my 1.5.1 upgade; a custom html and aa search form widget in the header section, and a blog display widget in the Aside First section, at least that I'm aware of so far. When I access the widgets config in the admin dashboard, though, the widgets are there and editable.  

Anyone else having this issue, and/or any tips on troubleshooting?

Thanks in advance!

Look at your app_data\logs.