You still have the problem of securing the encryption key if you want to decrypt in your code. If you don't need to re-encrypt in your application and only decrypt once when you start the application then asymmetric encryption using
http://www.basic4ppc.com/forum/addit...html#post11986 can be secure.
You could make up a string including the IMEI number and sign it with a private key then verify the signing in your application with the public key. With only you knowing the private key the string can be in clear text because if anyone tries to alter the IMEI number the signing check will fail. However someone with the technical capability could still reverse engineer your application and change your checking logic to always succeed.