Multiple Blog Installs Under Subdomain (htaccess issue?)

I have my main blog installed under a subdomain – http://blogs.mydomain.com and I’ve installed a couple new wordpress installs under the same subdomain like this – http://blogs.mydomain.com/blog1, http://blogs.mydomain.com/blog2, etc., figuring in the beginning that I would continue to install additional WordPress instances (using seperate MySQL DBs) in new directories under the subdomain.

Problem with my .htaccess file?
If I leave my permalink structure (under the Dashboard/Options area) as the default and choose not to have it be “Date and Name Based” it works fine. My preference is to make the permalinks date and name based, but when I do this and click on some of my links in the blog (feeds, About page, etc.) I’m getting 404 errors.

I assume this is an issue with what I’m including in the .htaccess file and I’m not taking into account locating my blogs under a subdomain. Can anyone offer suggestions on any changes I need to make to my .htaccess file?

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog1/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog1/index.php [L]
</IfModule>

Thanks…