FYI: I have subsetting working for TrueType fonts (fonts with TrueType outlines as opposed to PostScript outlines). Will be in next release. I'll look into adding support for PostScript outlines as well, but that might be too much effort, since most fonts are available with TrueType outlines. Annoyingly many open source fonts use PostScript outlines.
The code is totally new and undoubtedly has lots of bugs. But it seems to achieve a reduction of font sizes to about 30-40% of their original size for most books. f course if you use a huge font like Arial Unicode MS then it will be reduced to ~3%.