I'm using a childtheme of gridmarket with marketpress. From my main site, I need to be able to search across the network of sites to return product results. I tried your global search plugin, but it didn't work - I believe b/c of marketpress' custom post type.
So I decided to sort of fake a global search - I installed WordPress MU Sitewide Tags Pages to aggregate all products(a new site was created which is where the aggregated products are).
I modified searchform.php on my main site to search the new site created by thesitewide tags plugin (I changed the action attribute in searchform.php to get_site_url( 'new blog ID' '/') instead of home_url('/').
This turns up the search results as expected, but the permalinks in the search results reflect the new site w/aggregated content, not the site the product originated from.
The plugin author suggested that I change the theme of the new site which aggregates the products to TwentyEleven or TwentyTen, but that didn't work.