How to make Gevey SIM Work in iOS beta3
VERSION 3

IMPORTANT: You need to have preserved your baseband before doing this. Apparently, the only way to do it
 at the moment is using Sn0wbreeze by ih8sn0w. I tested this on baseband 04.10.01, but it should also work 
on 02.10.01 and 03.10.01. If you upgraded your baseband to any of the basebands on iOS5, you can still revert
 it by downgrading to iOS 4.3.3, which Apple is still signing, so no need for TinyUmbrella. This process will 
downgrade your baseband to 04.10.01.

- please note that I cannot assure this will work for anyone, but it worked for me-

STEPS

1- Remove the sim Tray from your iPhone
2- Turn it off
3- Insert Gevey SIM
4-Turn on (if tethered JB, just follow the normal steps using Redsn0w or iBooty)
5- The GeveySIM message will appear
-so far, the steps have been exactly the same as you would follow in previos iOS,  but here comes the little trick-
-UPDATE 1-
-also note that putting the gevey sim while the iPhone is off is in most cases crucial-
-UPDATE 2-
-in case the gevey message doesn't pop up, you can manually bring it up by going to 
  settings~phone~sim applications and then choose your language, this will show the usual gevey sim
 message and after you dismiss it you can go on with the next steps-
6- Dismiss the message, but this time, you WILL NOT see the 1 bar signal show
7- Wait until NO SERVICE shows (about 5 seconds after you dismiss the gevey message)
8- Call 112, hang up approx. after 2 seconds
9- Go to settings, and toggle airplane mode (on/off)
10- Sim failure will pop up
- note that INVALID SIM will NOT show, you will have to wait around 10 seconds after you dismiss the Sim failure pop up-
-UPDATE 3-
-If you wait for too long (after 'carrier' shows) this will no longer work and you will have to start over from step 1. 
11-Toggle airplane mode again(on/off)
12- Sim failure will pop up again, Carrier menu will show, and it will start searching for a network. 
After around 15 seconds, it will find it and you're ready to go.
-UPDATE 4-
Don't give up if it doesn't work the first time. It took around 3-4 times for me to make it work.

----------------------ALTERNATIVE WAY------------------------------
In case GEVEY SIM message does not appear after you boot your phone follow these steps:

- While your phone is ON, remove the sim, so you will get the NO SIM message
- Turn OFF your phone (DON'T INSERT THE SIM YET)
- Turn ON your phone and wait it to boot. "No sim" message will appear.
- Insert your Sim tray with gevey sim
- Wait some seconds till the gevey sim message pops up and dismiss it
- Wait for "NO SERVICE" and Dial 112. Hang up after 2-3 seconds.
- Toggle (on/off) Airplane Mode. "Sim Failure" message will appear, Accept and Wait for 10 seconds.
- Toggle (on/off) Airplane mode again. Another "Sim Failure" message will appear, now wait until "NO SIM"
 message appears (may take some seconds).
- Toggle (on/off) Airplane mode again and wait for your Signal and you're ready to go!
Thanks to @jppavez for this method.
-----------------------------------------------------------------------

Please let me know if this works for you!
@deefilippi on Twitter