In any promotional things I did not notice anything about GV integration. I am the maintainer of Dialcentral and that is available in testing form only for the n900. Contact me or join the discussion on the Dialcentral thread if you are willing to take the risks of enabling premature software by enabling testing. I am working on another project called The One Ring which is GV integration for the n900. It is still in heavy development and not ready for any use outside of debugging. I hope to have it ready for testing within the month.
About Visual Voice Mail. This is one of my favorite features - the only challenges is I want it to be client based - eg the voice files live on the device itself. I used to use a service called callwave which would send them to you as an MMS or Email, with a voice transcript, caller info in the subject field (with the file attached). then i could listen to it at my leisure w/ or w/o a signal, and not have to wait for it to load ota. Does anyone know if this service exists? I have searched on google and this site.