I recently had a problem with a client where using the Reporting Serices add-in for SharePoint in a forms-based zone was not functioning as expected. We went through all the suggested steps - applying the cumulative hotfix, making the Default zone the FBA zone, and making sure that the indentity of all the relative app pools were domain accounts - but were still getting this error:

An unexpected error occurred while connecting to the report server. Verify that the report server is available and configured for SharePoint integrated mode.

You might also see some more detail about a user (your FBA user) not having sufficient permissions. The Reporting Services web service shouldn't know (or care) who the FBA user is though - that's for SharePoint to deal with.

The reason this was happening is that the web service had anonymous access enabled, thus allowing SharePoint to try to pass it the FBA user's credentials, instead of using the trusted account to authenticate. Disabling anonymous access will fix the problem.