I have recently updated my Dell Streak via the official OTA update to Froyo, bad experience though. Of couse, the AT&T ROM will hide the option for side-loading an application.
The previous method to overcome this is to dial "*#*#307#*#*" which will then install the Dell Streak Engineering applications including "EMList". The EMList application has the option to enable the option "Unknown sources".
But the 2.2 update has made the installation of EMList not as easy as before. The installation via "*#*#307#*#*" seems to require a "PASSPORT" in SD card.
However, thanks to various Dell Streak forum (I am not sure who is the original source), the installation of EMList still can be done, but now requires ADW.Launcher. Once ADW.Launcher is installed and started, go to the home screen and add a "Custom shortcut". The reason to use ADW.Launcher is for the "Custom shortcut" option, which is not available in the default launcher.
Add an Activity and look for EMList and choose the EMListInstaller.
Click on the newly created EMList icon and it will briefly show a blank screen indicating the installation of EMList application.
Now look for the EMList application in the launcher and launch it.
Scroll until the end and select "App Security".
Enable the "Update Unknown Resource Property".
Now go to settings and the "Unknown sources" will be available under "Application".