[Eikinskjaldi]

Note: none of this is tested

Try this. Don't bother with a rewrite condition.
RewriteRule ^/([^/]+)/$ member.php?username=$1

This following example may not work because I don't know if mod_rewrite supports look ahead assertions, but it is a bit more robust (it doesnt try to convert php urls)

RewriteCond %{REQUEST_URI} ^/([^./]+)/$|^/([^/]+\.(?!php($|\?))(.*))/$ [NC]
RewriteRule ^/([^/]+)/$ member.php?username=$1

MyExampleForum.com/member.php?username=Username
if you can make it so i can get there by going to lets say:
MyOtherDomain.com/username

RewriteCond %{HTTP_HOST} MyOtherDomain\.com [NC]
RewriteRule ^/(.*) http://MyExampleForum.com/member.php?username=$1 [L,R]