Hopefully Sony will add this much-needed feature. In the meantime, you can use Womble's Mpeg Video Wizard 2003, or MPEG2VCR. You can also use TMPGEnc's Cut/Join tool, although it is pretty crude, and doesn't always work well with VBR MPEG.
As it stands right now, the only reason I might want to do this is to cut commercials from future Tivo transfers (that's if I use the original mpegs). If I "capture" via firewire, then all is moot anyway.