Here is an interesting chart comparing the iPhone with 3 other devices (click on chart to see larger version):
The chart is posted by Casey at maisonbisson.com, and you can see his commentary and links about it here:
Apple iPhone vs. Internet Tablets
PC Magazine has an article comparing rival cellphones:
Comparing the iPhone Alternatives
The have a detailed chart comparing 5 rival phones.
You might also like to check out Forbes.com:
Nine iPhone Alternatives
At Forbes the link will take you to a commercial window first; don't click on it, just wait a moment and do nothing, and it will transfer you to the article. If you don't want to wait, there is a "skip welcome page" link at the top of the page.