Over at Windows Phone Hacker, developer Jaxbot has figured out a way to enable "instant resuming" on Windows Phone 7.

Of course, you'll need a developer unlocked device and as they state there are some "unintended consequences" (though we don't know what those are exactly, we imagine it's memory/resource related). By using a registry editor, you can navigate to:

HKLM\Software\Microsoft\TaskHost

and set "DehydrateOnPause" to 0 (defaults to 3)

The hack takes effect immediately and as can be seen above, it makes starting/resuming any task or game nearly instant as it disables 'Tombstoning'. In fact, this feels like multi-tasking, allowing you to "go back" to the last few apps, having instant access to them. Of course the downside is you can't shut down any app either--we imagine eventually the system will force close something. Anyways, it's easily undone if something goes awry, till then, we'll leave it on and see what happens.

Source: Windows Phone Hacker