dotMobi, the registrar for the new .mobi top level domain now has a developers page at dev.mobi with quite a few resources for mobile web developers. The page has both mobile and PC versions. There are forums, articles, howtos, links to various mobile emulators and to browser vendor's developer sites and a brand new Mobile Ready Validator which is at mr.dev.mobi.
Like the online validators I wrote about here, The MobiReady one checks your page for valid markup, but it does much more. There are 25 tests including page and image size, use of the mobile friendly features such as accesskeys and default input methods, and avoidance of mobile un-friendly elements like frames, nested tables and pop ups. I find the validator very useful even though I don't agree with all the .mobi recommendations. I don't think that external stylesheets should be mandatory for example, as embedded stylesheets generally produce faster page loads with today's browsers. You don't have to be a dev.mobi member or even have a .mobi site to use the validator so give it a try and see if your site is MobiReady.