This thought never crossed my mind till today, so here goes.

Consider i want to optimize for the keyword 'webmaster forum'(assumption).

For a domain, to reap the EMD benefit it would be ideal to have something like
Code:
http://WebmasterForum.tld
.

For a folder or file, the ideal way to get the onpage benefit of 'keyword in URL', i'll have to have the URL as
Code:
http://www.domain.tld/Webmaster-Forum
However for Subdomains, which is the best way to have the keyword in it??
1) Seperated by a hyphen as in :
Code:
http://Webmaster-Forum.domain.tld
Or

2) No keyword seperation as in :
Code:
http://WebmasterForum.domain.tld