As promised, we're going to start the process of deprecating the notifications.sendRequest API function. The first phase of this starts tonight, as we roll out the new <fb:request-form> FBML tag.
Moving forward, the plan is to remove the notifications.sendRequest API function once we're satisfied that the majority of desirable request/invitation use cases are covered by FBML tags and that developers have had enough time to migrate (we're shooting for around 2-3 weeks from now).
The goal of this new system is to provide a flexible, consistent interface through which users can send requests and invitations, while at the same time saving developers the trouble of tweaking out this UI so that you can focus instead on building cool applications. For more information about how the new system works, please check out the wiki. If you have any feedback about the new tags, please leave it on the wiki's discussion pages - we'll try to keep an eye on them.