Should the common usage "Webmasters" be gender neutered into a separate webmistresses to describe female web site admin professionals?
Solution 1:
There is a strong tendency in recent English to eliminate gender-specific words. For example, most female actors do not want to be called 'actresses' (though there are a few that do want to).
For this reason, and because of Claudiu's point as well, I would suggest not coining any new words in "-mistress".
Solution 2:
Webmaster, to me, can mean a person of any sex. Additionally, 'mistress' has the connotation to me of an S&M mistress, or a concubine, etc., not of a female master, so saying Webmistress sounds pretty strange to me.
Solution 3:
In my personal experience working at various web companies, both males and females were referred to as webmasters. Regardless of whether 'webmaster' has gender connotations, the term is becoming obsolete, so this will probably end up a moot point over time.
Solution 4:
As Kosmonaut said, "webmistress" is the complete opposite of gender neutral.
There is an actual trend toward gender neutrality in occupational terms: flight attendant instead of stewardess, mail carrier instead of postman, that sort of thing. There is also some tendency toward avoiding the -ress form of some occupations; actor for both genders instead of actor/actress is the most prominent example, but people do still use actress.
Solution 5:
If you look at the International Webmasters Association website I think you'll find they're not an exclusive group.
The dictionary defines master as :
a person with the ability or power to use, control, or dispose of something:
a person eminently skilled in something, as an occupation art, or science:
I think it's fairly clear that a webmaster is not a gendered term.