Wow. It always amazes me how people try such crazy things to make feed URLs.
1. If you're using non-default permalinks, feed url's can be made by simply appending /feed/ to the URL:
2. If you're using default permalinks, feed url's can be made by appending feed=rss2 to the query string:
That's the only trick to it. This works on all pages, on all WordPress sites. It's been the correct way since WordPress 2.0, and it has not changed at all in that time period.
If you're doing this in a theme or template, WordPress can generate these for you and will display the correct one based on your current permalinks. Look in the link-template.php file to find all these functions: get_category_feed_link, get_author_feed_link, get_tag_feed_link, and more.
@stonegauge: See how easy this is? http://www.tampablab.com/category/blab/feed/