Are you planing on writing the "sqllite" one or are you talking about creating one to get the google contacts? It seems to me that if the goal is to get the google contacts that you have already accomplished that with TOR. The telepathy service is all that we really need for the n900. It seems to me that writing a "qt mobility contact backend" for this would probably create more problems than solve, writing an application to use more than one backend at the same time and have to merge them would only make matters more difficult.
If you are planing on writing the "sqllite" (or even better a generic sql one) we could try to work together. I would say if we:create a new sql "backend" with all the good features create some "plugable" way of adding/editing contacts so people can create other "plugins" (e.g. synchronize with Exchange, ...) create on plugin to get the "information" from the different "telepathy" accounts optionally one to synchronize the information with the "current n900 address book". we would be able to create a pretty powerful "qt" based solution to handle "contacts" - I would even go further and say that something like this IMHO should have been (and could become) the base for meego.