How do I change the title tag on the main page of my wiki?
This might not be what you're after, but you can remove/hide the title of the Main Page by adding the following code to either [[MediaWiki:Common.css]] or [[MediaWiki:Monobook.css]]:
h1.firstHeading { /* remove Main Page title */ display: none !important; }
Then you can add a level 1 heading with your custom title:
= Front Page =
You could also #REDIRECT to the page with the desired name (that's not an optimal solution though).
I believe it's also possible to change the default page from Main Page to whatever you like - search the list archives; it's been discussed before.
HTH.
-- F.