On various projects I've been using the excellent ASIHttpRequest library as a way to talk to the network from both Mac and iPhone applications. If you need to access web services from your app, ASIHttpRequest does all the heavy lifting for you, and leaves you to get on with writing more interesting things. It also has native support for some specific services like Amazon's S3. Like I say, it's a real boon.
One thing it lacks though is support for OAuth - which a lot of web services require these days. Back in the summer I made a stab at adding OAuth support to ASIHttpRequest, and got something working that was good enough for me, but not for general release. I've finally found time to try and fix that!
Over on github I've just put up a fork of ASIHttpRequest with OAuth support. Ideally I'd like to try and get this pulled back into the main tree, though I think it needs a bit more testing before that happens. That said, I've been using this code for a while, so please give it a try and let me know what you think (or fork my code, change it, and send me a pull request :).