The main thing for people who are wanting to root their phone is that this update changes HBoot to a new version and will therefore be incompatible with the current AlphaRevx S-Off software.
Other changes (where reported by two or more people):
Reports suggest it fixes the 'numbers not names' SMS issue, where numbers appear on text messages instead of contact names.
A couple of people have reported an improvement in the processing speed of the phone. Out of all the people who've received the update, it's noteable that very few have remarked on this.
The addition of a dock mode. [I have Dock on mine already.]
The addition of Picasa and LinkedIn to HTC Hub.
More unspecified bloatware reported on Vodafone UK.
As always with these undocumented updates, you can bet there will be lots of reports of people seeing something new that was there before.
Take these reports with a pinch of salt until the numbers build up or HTC publish a change log.
N.B. The update isn't available to me yet, so I can't independently verify any of the above. The only thing that is certain is the new HBoot version.