I think I've come to a personal conclusion on where I'm going to spend my open source efforts : enterprises and smaller service providers. Coming from Versatel, I can see how even a company as excellent as they are have an uphill climb against the much larger Tier 1 vendors who supply to the carriers. Traditionally, the enterprise was all about channel, and if you didn't own the channel, you couldn't win the war. My bet is that the channel is now the Internet itself, and the days of the stupid phone installer are numbered. Instead, we are coming to the days of the stupid system administrator, and he goes to the Internet first.
In the very long term, the large service providers may simply be the ones that supply the best pipes, as sustainable differentiation becomes harder to maintain. It will be up to the smaller providers to come up with the niche applications that are compelling and add value. Adding to that, I know that the PBX is not a function that's going away anytime soon, and I wonder if any PBX vendor can win a long term battle against Chinese phones and asterisk.
That said, I think I'm staying with the smaller service providers and enterprises.