brinked - 12:31 am on Dec 8, 2010 (gmt 0)
Unique meta descriptions used to be very important for getting more pages in the main index - but that was before Caffeine was deployed. I haven't tested it since then - but my sense of it is things have shifted.
I do think it would be better not to have any meta descriptions at all than to serve a whole lot of duplicates. But especially at the scale you are describing, you're probably much better off the actually write unique descriptions. It's not that big a job, just big enough to be a bit of a PITA.
Google says they do not use meta descriptions to factor in rankings. Either way, this is what people see in google that makes them either click or not click your site. This is your sales pitch to entice potential visitors, so make it count, a great meta description can go a very long way.
I would advise against automating it. WHat I found is the easiest way to go about this is to setup a instant edit front end solution using jquery this way I can go through my sites front end, click "edit" and put in a detailed description for that page that I want people to see in the search engines. This can also be useful for page titles if you have a lot of pages on your site that are generated from a script that you want to customize as easily as possible.