1.  When installing an application I get an error saying “for security your phone is set to block installation of applications not sourced in the Android market”?
Go to Settings > Applications and tick the Unknown sources box.  Press OK when the “Attention” box appear and this message will no longer appear.

2.  What’s currently the best way to root the Desire?
I strongly recommend using Unrevoked.  I posted up a couple of tutorials, one using Windows and the other using a Linux Live CD (I prefer this method).

3.  Can I Play DivX Videos on Froyo?
Yes.  There was a rumour that DivX playback would be supported in Froyo but it doesn’t look like it is.  However, you can download RockPlayer from the Market which is capable of playing DivX and many other video formats flawlessly

4. How Can I Move Apps to SD Card in Froyo?
Go on Settings > Applications > Manage Applications.  Under “Downloaded” or “All” select the app you want to move.  Click the button labelled “Move to SD Card”.
The above method works for apps which support apps to sd.  If you’ve rooted your phone you can use the ModInstallLocation which allows you to force all apps to be installed to SD card.  Alternatively you can use a ROM which has the unofficial A2SD and use that by creating an ext partition on your SD card.

5.  How Can I Delete Files on my Phone?
You can either connect your phone to computer via USB, set it in Disk Drive mode and manage all your files from the computer.  Or you can download a File Manager application such as FileGo, Astro File Manager or EStrongs File Manager and use it to manage all your SD card contents directly from the phone.

6.  How Do I Manually Close an Application?
Go to Settings > Applications > Manage Applications > Running.  Select the application you want to close and press “Force stop”

7.  Does Unrevoked work on Linux?
Yes, in fact I’ve written a tutorial on how to run it on Linux here

8.  What Partition Sizes Should I Use When Formatting To Ext?
I have an 8GB card and have found 2GB for the ext 2 to be plenty.  Swap space isn’t really required and can be set to 0, though I keep it set to 32MB.  The remaining 6GB seems to be enough to hold all my data for now, which includes photos, videos, ROMS and ScummVM games.  I’ve had the 2GB partition for over 10 months and haven’t yet come to filling it.

9.  Where Can I View A Changelog for Froyo?
There’s no official changelog available that I’ve heard of.  However, the folks at XDA-Developers have compiled a massive list of all the changes that are included in Froyo which you can view here

10.  When Will Froyo Be Available in Asia?
No date is confirmed yet.  If you got your Desire shipped from Europe to Asia then you should already have Froyo.  If you bought it in Asia then there’s no news on a release date yet.