Overiding the_excerpt

Greets:

This thread on edublogs got me thinking. Since many theme designers use the_excerpt on their category, date, monthly and tag pages, that will throw folks off as they;re expecting to see the entire content. (Not to single out the thread on edublogs or the poster. I could point at dozens of threads on wp.com.)

Now from an SEO position, using the_excerpt on those pages is a good idea so not to have an excess of dupe content. On the other hand, displaying full content is a better idea as noted in the edublogs thread on occasion.

Editing every theme isn;t gong to happen and there's no easy way to make a switch to go back and forth.

Is there an easy way to create a filter within a plugin or other method to tell wp to use the full content of the Post within the_excerpt?