I believe you are looking for the about:blank trick. Check out this post for details: http://forums.adobe.com/message/2786957#2786957
Does Flash Builder "Burrito" support cross-compiled debugging?
I'm not sure what cross compiled debugging is, could you explain a little more?
Debugging a swf that has gone through the Packager for iPhone and converted
When you compile the IPA you have the option for ipa-debug. When the app is
opened, you are prompted for an ip address of the machine you are debuging
on. In Flash CS5 you'd go to Debug > Begin Remote Debug Session > AS 3.0
and you'd see all traces and what nots.
Just wondering if possible in Flash Builder.
I don't believe it is currently a supported workflow in Flash Builder, but if you use the output debug IPA from Flash CS5 you should be able to use the same about:blank trick to connect via Builder.