I will let google answer to your question:
http://googlewebmastercentral.blogspot.com/2008/09/dynamic-urls-vs-static-urls.html
In the article:
Which can Googlebot read better,
static or dynamic URLs? […]While
static URLs might have a slight
advantage in terms of clickthrough
rates because users can easily read
the urls, the decision to use
database-driven websites does not
imply a significant disadvantage in
terms of indexing and ranking.
Providing search engines with dynamic
URLs should be favored over hiding
parameters to make them look static