The guys at Ausdroid have got a rundown of how to update your Optus Dell Streak to Froyo. It's not an official update from Optus though, and it may delete your phone's APN settings, but given the seemingly neverending delay for Froyo updates to Android phones, a few APN settings isn't going to hold you guys back.
To get the update, first make sure you take note of your phone's APN settings so you can update them manually after the update. Then point your phone's browser at Dell's Mobile Update page, mobileupdate.dell.com/?version=GAUSB1A110100. Follow the instructions and Bam! You should have Froyo. You can then check and update your APN details if they disappeared during the update.
So far, commenters on the AusDroid post seem to say that the update is relatively painless, but naturally, do it at your own risk. And make sure you backup all your data before you attempt the update.