Forum software with public API

Hi,
This is slightly off-topic - but only a little bit. I'm sure someone here will be able to help.

I have a client who I am helping with some integration issues.

Basically, they are building a Flash App (AS3) which users will fill out to register with my clients services. The requirement is that when this Flash App form is submitted the user is added to a primary database AND also to a newsletter subscription AND to a forum.

Obviously, Campaign Monitor's API will make the newsletter subscription easy, but I cannot find any forum software that supports a public API. Can anyone suggest a system (open source or paid license) that might support a public API to allow a remote application to add a new user, in a similar fashion to Campaign Monitors

The client is pretty keen on phpBB, which has an API, but it would all have to run as PHP on the same server - which is not possible in this case.

Can anyone help with a recommendation?

Thanks.

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