Usually, I've seen it with forms, but I've found it helpful to group related sets of data (eg when you have multiple tables on a page, using a fieldset around each table or group of related tables to define a visible meaning and a group name (legend)). Is this abusing the fieldset tag to the point where, in my uses, it no longer has semantic meaning?
I believe this would be abuse. http://www.w3.org/TR/REC-html40/interact/forms.html#h-17.10 states "The FIELDSET element allows authors to group thematically related controls and labels".