In game music and in game messaging (through your PSN friend list) are already possible. Two little "plugins" (If we may call them that) were developped by Sony and have been released, so some games such as Resistance Fall of Man feature in game messaging (sadly only through the online main menu) while others such as Super Stardust HD feature in game custom music for all levels. These applications should be able to work from anywhere withing the game too so I doubt that it could be a problem implementing them into the Pause menu of WipEout HD or something, as well as in the main menu. What's missing is obviously the live chat (with microphone) that is available in the XMB but this may be a little harder to implement because it has to use the internet too, with the game running on the net as well (While the others do not require such resources).
Also, features such as password protected games are already available in PULSE and with the in game messaging I talked about earlier, you could theorethically send a message to one of your friend saying: "Hey, I just made a WipEout HD game, come and join it! The password is "chuck norris" without the " " or something like that ;) However, speaking of Live chat earlier, while the XMB's standardized interface may not be available in game, it doesn't prevent the game from having it's own voice chat. Also, WipEout HD could feature it's very own "Buddy List" (Again like RFoM) which would allow in game invitations to races without the need to resort to the PSN friend list and messaging systems.
All in all, there are many ways that the core features of the online game could be available to us in WipEout HD, so I doubt that they could be the reason for the delays. What Colin said about the game being 99% complete might mean that there are still some stability problems while playing online. Then, the server side must be complete too, and the marketing team must be ready to publish a firm release date and start doing their job like they should be doing.