I noticed a Sony person mention the update in relation to a network-rendering question. This is different to the version shipped on the DVD's that we picked up at NAB (I believe) so it will be a good idea to install it I think.
[edit] Just checked and the version I installed from NAB was 5.0 (build 122) and the version I just downloaded and installed is version 5.0a (build 134).
v5.0 was the released boxed version, the same day that shipped v5.0a was released, and if you downloaded it from the website, that's what you got. So how's that for quick updates? Same day :-)
I don't know all of the differences between v5.0 and v5.0a, I only know there were some minor bugs that where fixed that didn't make it in time for the v5.0 boxed released version. Usually they put those differences in the Vegas release notes, on the updates page, but it looks like that information didn't make it.