[UPDATED for ios7 beta] “We’re unable to complete your activation”, solution for those with the Apple developer program [UPDATED for ios7 beta]

[update 11-06-2013] I ran into issues with ios7 beta where my newly registered device was not recognised. I tried a number of things e.g. activating via a different connection instead of iTunes, downgrading back down via DFU mode. The only thing that worked was to do a Shift+Update (or Option+Update) from ios6.x then pick the beta dmg/ipsw, as opposed to Shift+restore. Somehow this appears to trigger an alternate activation process.
[updated 22-02-2011]
There’s an easier way, assuming you’ve sync’d your device at least once.
-on Windows 7, head over to:
C:\Users\[YOUR_USERNAME]\appdata\roaming\Apple Computer\mobilesync\backup\
there, you’ll find folders with 40 letters…and voila you’re looking at your UDID!
-on Windows XP, it should be somewhere around:
C:\Documents and Settings\[YOUR_USERNAME]\Application Data\Apple Computer\SyncServices\Local
and on a Mac:
/Library/Application Support/MobileSync/Backup
[end of update]
iOS 4.2 Beta1 is out. Wow I thought. I’ll just geddit and flash…
But then after the flash I was hit with “We’re unable to complete your activation” screen in iTunes. The solution, Apple says, is to register my device with the developer program- which I had not yet done.
How to register? By getting the UDID from the Summary screen in iTunes. And how do you get to the Summary screen? by getting past the Restore screen. And to get past the Restore screen…..you need to register and activate. what?
So seemingly I was stuck in a loop of fail. None of the Apple KBs helped. After searching up and down through huge forum threads, I finally found the solution. And here I share:
You need:
a Mac
your iP /ad /od /hone
and the sync cable.
all plugged and connected.
1, in OSX, go to Apple> About this Mac> More Info…
2. Contents> USB> iPad> Serial Number
3. and that’s it. There’s your 40 letter UDID.

Sponsored link:
Looking for the iPhone 4S?
Have a look on Amazon.