Dave Winer advances a theory on why there's no SDK for the iPhone right now:

There's one application, for sure, that could mess up not just Cingular's West Coast network, but the whole idea of an Internet-capable PDA with wifi that wants to be a conventional cell phone. It's called Skype, and it really worries the phone companies. So much so that they might have made the closedness of the iPhone a condition of working with Apple. Permalink to this paragraph

Shortly after Apple opens the iPhone, if they ever do, expect a compatible version of Skype to follow shortly after.

During a conversation with Daniel Miessler, it occured to me that:

There’s at least one VOIP provider who could still happily have an interface on the iPhone, despite the lack of an SDK: JaJah (http://www.jajah.com/)!

(yes, I corrected a typo I made when posting the original comment. So sue me.)

JaJah provide a service whereby you specify both the number of the person you want to call and your own number; it then calls both ends and joins them together. In effect, you have to pay for two VOIP calls - one from JaJah to you, one from JaJah to the other end - but that's usually cheaper than paying for one non-VOIP call from you to the other end.

JaJah are the only VOIP provider I know of who provide this functionality. It's almost too good a deal for them to ignore - They'd be insane not to have a nice interface ready for the iPhone as soon as they can.

PS. Can anyone tell me what unit of time JaJah's rates are based on? The rates page happily tells me that to call the US from my mobile will cost me 17.9 cents, but it doesn't tell me what unit of time that rate is for - per second? Per 30 seconds? Per minute? I'm assuming it's one of the latter two options, but I have no idea which one. Either way though - is the billing per-second, or per-[30,60] seconds? I can't find anything on the rates page, nor anything in their FAQ, that covers this.

Update: Question 11/15 in the Billing section of the FAQ does answer part of this: JaJah bills in one-minute increments. Still no word on whether the quoted rates are per-minute or some other unit though.