Well it seems that just like in the gaming world, where Microsoft's X Box Live has been racking up voice minutes for years, has seen Second Life roll out voice services to a point where 15 million minutes were consumed last year, at a rate of 700,000 users per month signing up. That's faster than Vonage and may be faster than Skype at this point.
With the ability to call and be called from your first life universe, basically what Linden Labs (the creators of Second Life) are doing is seeking to become a second Skype. Depending on how "open" the Second Life Universe is, if there are such things as API's it won't be long before Skype has a "Second Life" calling capability, or would that be Third Life once the IPO and make it so Skype users can call Skype 2L users directly, without ins and outs of the pre-first life PSTN network.
All joking aside, what this is about is monitization and connecting. In Second Life people want to communicate and VoIP is a proven way, ala X Box Live to do that.