Most webmasters who understands the basic of search engine optimization will tell you to avoid using dynamically created pages. The reason is that search engines often have difficulty in indexing dynamically created pages. However, according to the recent update on Google Webmaster Central Blog:
[Google now indexes] URLs that contain that parameter [&id=]. So if your site uses a dynamic structure that generates it, don’t worry about rewriting it — we’ll accept it just fine as is.
Keep in mind, however, that dynamic URLs with a large number of parameters may be problematic for search engine crawlers in general, so rewriting dynamic URLs into user-friendly versions is always a good practice when that option is available to you.
If you can, keeping the number of URL parameters to one or two may make it more likely that search engines will crawl your dynamic urls.
So if you already have a dynamically created website, you must make sure you only use no more than two parameters within the URLs. You may want to re-write your URLs using Mod Rewrite to ensure most the search engines especially including Google will be able to index your web pages.