I can't think of anything more important than SEO for an ecommerce site. Some of the easiest changes can pay off in improved search engine rankings. For example, using header tags <h1> by default in templates, which someone mentioned in a previous post.
Reducing viewstate is also helpful so that content comes earlier in html file. Viewstate could also be moved to bottom of page. I've seen sample code online using HTTP handlers that do this.
The URLs are good:
domain.com/Products/82-blackberry-bold-9000-phone-black-att.aspx
and
domain.com/Category/29-books.aspx
...and based on SEO principals they could be improved:
1) add categories into URL (this adds more keywords into URL and helps ranking)
2) put id number at end of product name (blackberry is a more important keyword than 82 and looks better)
3) don't use /Products and /Category if possible (those aren't important keywords); a page could be identified as category or product with a letter (such as p and c) before id number
-if urlRewriter needs a keyword, /Shop and /Buy are more likely to be entered as search terms in web search engines.
4) saving capitalization also makes urls cleaner
domain.com/Electronics/Mobile-Phones/Blackberry-Bold-9000-Black-ATT-p82.aspx
I haven't tried, but maybe some of this is possible with URLRewriter rules?