I too have a Droid Eris (unfortunately) and just realized that this phone can't delete large threads. I came from a BB Tour 9630, in which I didn't "have' to delete texts like I do with the Eris. If I wanted to delete my messages, I had no problems doin so. Some of my Eris threads have 1K messages, so I figured I should delete them, in hopes this turd phone would run a little faster. After readin this thread here, I figured I'll just get flamed for not keepin up with "housekeepin", but fawk it!
The solution I've found is to just spend HOURS deletin each text, one at a time.
I have now learned that besides gettin a better phone as soon as I can, I can't keep many texts.
Pyro