Pex just posted on the official forums:
We are planning to launch the Speedway Thunder update this evening. At the soonest, it will start at 10:30 pm and last between an hour and an hour and a half.
If we run into issues, we will delay the update. If it doesn't happen this evening, you know we'll get to it as soon as we are able.
Update notes are published the morning after the new content is live.
Thanks for your patience!
Woot, let's get ready to race!
An update on this, from Pex at the official forums:
We have had to delay this update until Tuesday. As soon as we have a timeframe, we will let you know.
Another update around 5:30PM PST on 07/28/2009:
Bad news, folks. Looks like today is not the day.
If all goes well, it will be tomorrow (Wednesday) evening at or after 7pm Pacific Time.
All the more time to practice your kart skills!