My mate and local web designer, Oliver Ker, is a long time fan of a particular flavour of CMS called TextPattern. I tried it once for a project but for some reason I found ExpressionEngine easier to get my head around. Anyway, for the last few months (or possibly longer, I'm not entirely sure) he's been hard at work creating a mobile version of the admin interface allowing you to write and edit posts on the fly, right from your phone.
I got a quick preview last week and I have to say it's looking rather nice. The interface is really clean with simple navigation which gives you quick access to the sections you need throw in little touches like an expanding text area for long blog posts and more, it's something you should try out for your self.
At the time of writing it's still in the testing phase although I think he's been sending early previews so I imagine it shan't be long until it's fully released. If you want to know when you can get your hands on it, head on over to the site and send him your email address.