The Apache Cordova Project recently released Cordova 3.5.1, a patch for Cordova Android only. It contains a few very important security updates. On PhoneGap Build, rather than add 3.5.1, we’ve patched our Cordova Android 3.5.0 release with the security updates. For more information on the security updates, check out the Apache Cordova blog post.
To summarize, building with PhoneGap (Cordova) 3.5.0 on PhoneGap Build will in fact use 3.5.1 for the Android build.
If you need more info on this update, just ask.