As WP Subdomains appears abandoned and people are reporting compatibility issues with WordPress 3.1, I think this is a safe time to make this improvement.
Apologies to anyone still using Dave's WordPress Live Search with WP Subdomains on an older version of WordPress, but this change is needed for my plugin's stability.
For the technically-inclined, I'm replacing my code for building URLs with a call to WordPress's plugin_dir_url() function. This is a function in WordPress core. Plugins which want to change how this function builds URLs (say, to force a different subdomain) need to implement the plugins_url filter and alter the string it passes.