I've also been migrating of my music library from WMV Lossless to AAC Lossless. I initially tried to drag a big folder full of nested album folders containing WMV Lossless media from a networked drive (SMB) over to iTunes on the Mac, but it did... absolutely nothing. So now, my $700 Dell is cranking away at 95% CPU on both cores, converting a list of more than 5,000 songs, many of which are long, classical tracks. It is maintaining a conversion rate of 11.8x, which Victor says is pretty fast.
At 3am this morning, iTunes had converted more than 3,000 songs in 26 hours when Windows forced a reboot due to software updates. ARGH! So I had to dig around to find the 2,000 songs hadn't been converted yet and drag them over again. Needless to say, I have disabled Automatic Updates.
Keeping both lossless and compressed versions of music is not easy. I was telling Victor last night that I wished iTunes supported "stacking" of music not unlike what Aperture does with photos. My main music library should be able to track compressed versions alongside lossless versions, or at the very least, have an option to auto-compress when sharing music or transferring music to portable devices (like what iTunes does when you attach a Shuffle). What I want to do is to have access to my lossless media at home and my compressed music on the road.