Here's some more information.
Surf-contro/surf-watch has letsrun.com in its list, categorized as sports. If your employer/school/etc bans "sports" in their banned file, letsrun.com is gone.
Websense, a huge vendor, does not seem to make its list public. However, there is a form on their site to let you suggest a site be removed, if you think websense is a problem.
N2H2/Bess/Securecomputing also has letsrun.com listed in its sports list. If "sports" is selected for banning, it will be gone.
Etc.
Some products, such as dansguardian, actually search a page for pre-defined terms or bad words. The filter creates a score for each page. If the score exceeds a threshhold, the page is banned.