I think I've found the solution. CS2 + Phonegap Build for Android:
Export the project to Cordova.
Then in the config.xml remove this line:
<plugin name="com.mcm.plugins.androidinappbilling">
...Instead add this:
<plugin name="cc.fovea.cordova.purchase" spec="~7.1.0">
<variable name="BILLING_KEY" value="YOURBILLINGKEYHERE" />
</plugin>
...you don't need to bundle that plugin with your zip file for Phonegap Build as it already uses it.
The problem seemed to be that Phonegap Build no longer supports the com.mcm.plugins.androidinappbilling plugin.
Seems to work for me. Let me know how you get on.