You can set it to Apple_Boot using
gpt and this is indeed what hides it from GUI apps like disk utility. Apple_Boot is just a hidden version of Apple_HFS; the recovery partition is just an ordinary journaled HFS+ volume that can be manually mounted with
diskutil mount disk0s3 (or whatever slice the recovery partition happens to be).
To change a GUID partition type to Apple_Boot one needs to know the UUID -- which for reference is
426F6F74-0000-11AA-AA11-00306543ECAC -- and remove the partition using gpt then re-add it with the desired type. This won't destroy the data of course, but one needs to make a note of the
start and
size values of the partition (as shown by
gpt show) before the remove and then use them when re-adding. See the manual page for gpt.
(*Note: the UUID of the recovery slice can be seen on a Lion system that has said slice by issuing a sudo gpt -r show disk0 command; I didn't just pull that one out of the Air... or did I, since it came from my MBA? lol)
FWIW the recovery slice of a Lion install can be copied to any other slice that's 650+ MB by simply using
dd. The target partition can be created and formatted using the GUI Disk Utility, then the type changed to Apple_Boot using gpt, then the contents of the source recovery partition dd'd to the target. I dropped my MBA4,2 recovery slice on an external USB disk and booted my wife's 2010 Macbook with it (which I didn't expect to work, TBH).
Lastly,
here's a post and corresponding thread on some weirdness (most likely yet another bug) in regards to how Lion's Disk Utility treats that recovery slice in the case of erasing another slice.