This seems like a very nice and useful plugin, but please fix some basics.
1. Work with caching tools. WP is a very convenient CMS but a pathetic architecture with query_posts and whatnot. Hackjob, mostly. Which makes it vital to have caching plugins. I use the superb "1 Blog Cacher", which in fact allows me to skip the cached version of the site for browsers that match the strings in a variable called OBC_REJECTED_USER_AGENTS. This is handy, as I put some strings there such as "blackberry" (for my Blackberry browser). This works, but the manner in which MobilePress checks for blackberry is flawed. Or something. Please make sure your plugin interoperates with some of the top caching plugins.
2. Fix "eregi" type stuff in the code. This is NOT good for performance, as PHP's own manual will tell you. It's easy to replace this with something much faster, like the "stristr" function. (In system/classes/check.php). For simple blogs with a 100 or so posts and not many things in a page, this is okay, but for busy sites with lots of content, it helps a LOT to have optimized code.
Thanks.