A goof on Nintendo’s customer service site may have revealed a bit more than intended; however, it has since been removed.  Specifically, the revelation was found in a troubleshooting section regarding the redemption of Nintendo Points Cards.  The following was on that page:

To redeem a Nintendo DSi Download Ticket number, such as for the Nintendo DSi Speak Channel, enter the number off the Nintendo DSi Download Ticket in “Settings and Features” off of the main Nintendo DSi Shop page, then select “Nintendo DSi Download Ticket” (emphasis added).

Although Nintendo refuses to comment on rumors, this looks like the real deal.  There will most likely be a VoIP option for your DSi sometime in the future.