Step 1 - requesting an invitation
I requested an invitation on the day Google Music was announced at http://music.google.com/about/. The invite request form IP checks to ensure you are in the US, so you need to be connected via a VPN located in the US. I used http://www.privateinternetaccess.com (my VPN of choice as it's free with MoDaCo Plus).
Step 2 - installing the latest Google Music software
After requesting the invite, I installed the latest version of Google Music ready for when the invite arrived!
This can be achieved by either installing MarketAccess and spoofing your location as the US, or by installing this APK.
Step 3 - trying to boost my chances by spoofing a 3.1 equipped Xoom
Twitter and MoDaCo member @anditails mentioned that he had received an e-mail inviting him to Google Music as he had an Android 3.1 equipped Verizon Xoom. In fact, he has a UK WiFi Xoom that he had flashed with the 3.1 ROM. This got me thinking.
I edited the build.prop on my Eee Pad Transformer, substituting in the lines below:
# begin build properties # autogenerated by buildinfo.sh ro.build.id=HMJ37 ro.build.display.id=HMJ37 ro.build.version.incremental=124251 ro.build.version.sdk=12 ro.build.version.codename=REL ro.build.version.release=3.1 ro.build.date=Sat May 7 02:22:32 PDT 2011 ro.build.date.utc=1304760152 ro.build.type=user ro.build.user=android-build ro.build.host=android-test-7.mtv.corp.google.com ro.build.tags=release-keys ro.product.model=Xoom ro.product.brand=verizon ro.product.name=trygon ro.product.device=stingray ro.product.board= ro.product.cpu.abi=armeabi-v7a ro.product.cpu.abi2=armeabi ro.product.manufacturer=Motorola ro.product.locale.language=en ro.product.locale.region=US ro.wifi.channels= ro.board.platform=tegra # ro.build.product is obsolete; use ro.product.device ro.build.product=stingray # Do not try to parse ro.build.description or .fingerprint ro.build.description=trygon-user 3.1 HMJ37 124251 release-keys ro.build.fingerprint=verizon/trygon/stingray:3.1/HMJ37/124251:user/release-keys ro.build.characteristics=tablet # end build propertiesbefore doing a hard reset, re-setting up the device, running the Market, running Music and doing everything else I could think of to make Google see me. I left the configuration like this for a few hours then restored my normal backup.
Step 4 - the invite arrives
Lo and behold, a few days later, my invite arrived in e-mail!
I tried to accept it in the browser but it gave me the same error being outside the US, so I connected the VPN to accept. This is the last time I needed to use the VPN. I can now upload / download / play music on my PC and phone over a regular connection. And it's great!
I hope this might help someone get on... please post below if you have any success!