Thinking further about what Google uses for description: Google seems to use whatever best relates to the query term.
For example, I just Googled 'web design
seo' (not in quotes) and the description it uses for cyber-key.com is indeed my meta tag description. My site is listed in DMOZ twice, and I can't find a search query that prompts the use of that description.
And here's an anamoly:
When I do a search for 'key west web design' it uses content for the description, but takes the title from DMOZ. I've always found that curious. For most other searches it uses my title tag.
I also did a search for 'design, hosting and search engine placement services cyber' and the description reads: "
Affordable web design and top search engine placement firm in Key West, ... Cyber Key also designed, maintains, hosts or provides marketing services and ..."
"Affordable web design and top search engine placement firm in Key West," is the alt tag on the logo. There is no place else on that page where the word 'firm is used' ...
So, alt tags are occasionally used as descriptions ... I am liking that little discovery!
Cheers, MJ