The wordpress plugin does a good job adapting a wordpress site for mobile devices, however it doesn't use the XHTML Mobile Profile required by the .mobi registry.
My blog (not a .mobi domain), adapted with the Wordpress Plugin, is readable on my phone and the emulators I've tried, but only scores a 3 out of 5 (fair) on the ready.mobi test. My hand-coded mobile sites score 5 out of 5.