Further to Chad's post, here's the full announcement on www.ibm.com (worth a look as it's pretty comprehensive):
(I spy this link is also hidden away in one of the comments below)
TSM 5.5 is available for download for Passport Advantage customers as of today (Friday 16th of November 2007), and media is available from 14th December 2007.
There's also an IBM Redbook for TSM 5.5 being written at this very moment and there's likely to be a draft available mid-December (thanks for the info Craig).
Finally for now, it's worth noting that after a touch over three years in service, TSM 5.3 goes out of support at the end of April 2008 - time to exercise those upgrade muscles as I'm sure many shops haven't even taken the leap up to TSM 5.4 yet.
However, if this all seems too daunting too soon, I understand that IBM are offering extended support for 5.3 for two further years (at cost).
Over and out...