Many of us have been running the unofficial Android 2.1 update (build 6941) on our UK and US Dell Streaks since it was leaked a few weeks ago, but those who prefer to play by the rules will be pleased to know that O2 is expected to release the official update (build 8023) tomorrow, September 1st.
Funny thing, though, is that xda-developers forum member Greenarcher707 already got this update from official channels a while ago. He posted the file a few days ago and had been running the build on his Streak several days before that. In other words, there's no need to wait for the update notification to appear on your device; you can go ahead and flash the update manually now (at your own risk, of course).
There aren't any major improvements in this new build (some say it's a bit smoother, while others say it lags), so I'm personally going to stick with the pre-release build I've been running on my AT&T Streak until a Froyo update is leaked.
Note that the new update is technically for O2 Streaks only.[xda-developers]