Setting up .htaccess so that domain name masks subfolder (where wordpress is installed)

Hi folks,

I'm setting up a blog at http://www.davejmason.com. The wordpress install is in the subfolder /main so that I can install other applications on the server (in different subfolders) without any overlaps. This is because when I install WP in the root and then try to install another WP in a sub folder, plesk tells me I can't do this)

I did some googling and found that I could mask the /main part of the address so that when visiting httpdocs/main/ the browser would only display davejmason.com

The trouble is, this only works when I don't enter the www. - is this a DNS issue or htaccess issue?

How can I make this work?

Also, would using the domain like this:

a) negatively affect SEO?
b) mean I can no longer install apps in a subfolder (eg httpdocs/application) and have it display as http://www.davejmason.com/application rather than davejmason.com/main/application?
c) be pointless? Should I just install it in the root? I guess the only reason I'm doing this is to make sure that wordpress doesn't clash with any other app installs. If it's not going to anyway then I might aswell migrate it there right?

Here is my htaccess code:

RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?davejmason.com$
RewriteCond %{REQUEST_URI} !^/main/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /main/$1
RewriteCond %{HTTP_HOST} ^(www.)?davejmason.com$
RewriteRule ^(/)?$ main/index.php [L]