If you are looking for an iPhone application to make calls to Google Talk users (and all others who use Jingle extension for XMPP, or XEP-0166), check out Talkonaut. App is currently free on the app store, but I wouldn't be surprised if that changed; in fact, I'd love it if the developers made extra money by doing so. I'd give back the free version and give a few bucks for the paid version. It's worth it.
It even contains Jingle-to-SIP conversion; that is, you can use the XMPP connection to establish a SIP connection. SIP is a "more traditional" VOIP protocol, while Jingle is what Google Talk uses.
They also have partnered with many SIP-to-PSTN providers (where PSTN is regular telephony, such as landlines and cellphone), enabling you to make GTalk-to-phone calls for apparently very low prices. At the moment I have no need for such a service, but it might be a good way to pay back for the free app. I'm not getting paid to say all this — as a developer, I can appreciate the numerous hours that went into this project.