I've found ready.mobi and its family, you can test webapps with some emulators and there are some guides on mobiforge.
I'm using also my N78 to test my little webapps with nokia's browser and opera mini.
There is any other great resource for mobile webdesign?
Two great places to start with are Meagan Fisher's "Designing Effective Mobile Interfaces" talk from the FOWD conference, and Carsonified's "Coding for the mobile web" article. Both give a great insight into the differences of mobile development.
I haven't read it, but Cameron Moll wrote a book on the subject of Mobile Webdesign. Judging from the sample I'd say it's worth the few bucks.
For iphone in particular I find wordpress to be outstanding. Using wordpress you are able to build a solid website for normal pc browsers and re-direct the theme for mobile users.
- A great theme to start with >> http://www.bravenewcode.com/wptouch/
You could also check Dreamweaver, which I think has some defaults in it for rendering like mobiles.