Searching across multiple installations of WP

I am setting up a site that uses multiple WP installations on sub-domains to coincide with the main WP installation. Each blog will have a separate topicality, but all topics are related to the purpose of the main site; i.e. subdomain[x].domain.com or domain.com/subdomain will each have a WP installation. It will be paramount to the design of the entire site to have cross-search functionality to ensure that articles, posts and comments are visible throughout the entire site, i.e. “search-able” through the entire site from any of the sub-domains.

I checked the online documentation under “Installing Multiple Blogs” and found instructions « here ». I am contemplating using the single database approach, but in reading the instructions, it tells you to change the table prefix $table_prefix for each additional (or all) install to something other than the standard wp_.

From what I can tell, this will create separate tables within the same database, but won’t allow for searching amongst all the relevant tables without doing some serious MySQL re-coding.

Am I correct in my assumptions here? If so, is there a workaround? I’m not the best SQL programmer in the bunch, so the prospect of having to re-work all the standard search functions in WP along with all the relevant widgets/plug-ins seems daunting. If I’m not correct in my assumptions, do you have to do anything special to enable searching through all site content from any/either of the sub-domains?