Updated 7/4/2011
Windows Phone SDK 7.1 Beta 2 has been released to developers today. This post will act as a container for new SDK tips and tricks. I will be updating it as I learn new SDK intricacies. Since most people do not read the release notes it may be a good starting point. Here are the two items from the release notes that caught my eye:

• The task-switching user interface that is accessed on physical devices by pressing and holding the Back button is not supported on the emulator. To resolve this issue, you can click inside of the emulator to set focus, and then press and hold the F1 key on your keyboard to bring up the task-switching user interface.
• Microsoft.Advertising framework is included in SDK, but it does not show up in .NET tab when adding a reference to the project. The assemblies are located at \Advertising for Phone.
• Installing Mango update fails with error code  80180048. This will happen if you have “walshed” your phone, then later on tried to repair it running Walshed Phone Support Tool  without selecting ALL installed languages on the device. The key is to select ALL installed languages instead of only the active language as I did initially on my Samsung Focus (AT&T). The fix is to download the tool again and re-run it with all installed languages selected. After that you will be able to resume the update through Zune. Rafael Rivera has posted detailed instructions.
• Once Mango update has been installed, the device is developer locked, again. Running any of the side loaded apps will result in a message that the app was revoked and will be uninstalled. Run the Windows Phone Developer Registration tool to unlock the device in order to keep your side loaded apps on the device.