To follow up on the last post about mobile web design, here is a great link to design considerations related to the size of mobile phone display resolutions over at uxbooth.com
I really like the compiled list of resolutions to devices, which I've summarised into the following graphic (note that this is by no means a complete list) - click to enlarge at 100% pixel size.
The article also has a good introduction to CSS3 media queries.
The best bit of this post at uxbooth.com is the comments section - have a look at the comments about the iPhone 4 display resolution.
You'll notice on my graphic that the iPhone 4 (grey box) has the "biggest" display, however in reality this is the same as the previous iPhone (orange box) but with a higher dpi. The discussion in the comments section covers this in more depth (and how it effects web graphics).
Now you know about different screen sizes, you really need to know about viewports and how viewports relate to screen size and the all important viewport meta tag.