I just fixed my kite with this awesome new product from Airtime Kite.
It's called the "Valve Trap". Reuse your valve that fell off and be back in the water
as fast as you can get the repaired bladder back in the kite.
Here is a link on how it works!

http://24.media.tumblr.com/tumblr_mekhe ... o1_500.jpg

http://www.youtube.com/watch?v=18TybPNIoXA

My spin off idea for this product should be called the
"Valve Guard" stick these over all the valves that might fall off or leak,
to keep the air in the kite so you don't have to swim back to shore if you kite leaks out.
It just a Idea ???
Hopefully they will make bladders that the valves will never fall off or leak!
Cheers!

I really like the idea of this new product, make any valve replacement easier, faster and more secure with the 4" diameter surface. The "valve trap" is a good name too

Here's a writeup I put together on how I fix mine:
http://imgur.com/a/tBIcR#

I like this for when you have a valve that is easy to get off like the one in your pictures. But when you want to remove a valve that is more difficult to get off (even with boiling water) you will tend to get a damaged bladder area below the valve so in theses cases I prefer to simply cut around the valve perimeter and using the Airtime trap above is a great solution in that case and super fast if for example you are in a kite trip and need your kite fast! Also I often replace valve that I don't like (like the one you have in your picture) and install an other type so again the trap will be welcome for this job. I will order few for sure but like your way of doing it to reglue a good working valve in place. Does a perfect job.

Definitely using a PU sealant for fixing valves is the way to go.
KITEAID uses a similar sealant in its Bladder/Valve Repair Kit.

The problem with these self stick valves is that you have one chance to get it right. They use a very tacky rubber based adhesive that does not allow you the ability to re-position the valve base if a wrinkle is apparent. If you have a small wrinkle once the valve is installed, you will have a leak. It only takes 20 minutes for that small leak to make your kite lose enough pressure to end your session.

PU Glue is the way to go! It takes a little bit of time to cure but some things are worth waiting for!

Here is the link:
http://www.kiteaid.com/videos
I just went to the Kiteaid website and check out the how to videos.
Everyone I know has tried to use the wrong glue to try to put the valves back on!
Aqua seal, 5200, Shoe Goo, Etc. and nothing has worked.
My problem is that I live in a very humid environment and humidity eats the glue.
I will put Kiteaid VS. Valve Trap to a test to see who wins against heat and humidity??
If you leave your kites in your car on this island, (Saipan) they can get over 100 degrees just while you go eat lunch.
Thanks for the Second option!!

Excellent product. In the past I've glued mine back on but it takes about 8hrs (over night).
Be the thing to have on while on a kitesurfing vacation.