On the other hand, you're entirely right about Ethernet connectivity. Making a Basic Stamp tweet is somewhere between nontrivial and impossible.
Still, I don't think that the Arduino is miles ahead of its competitors. It's definitely sweet for small projects with limited I/O, storage, and bandwidth requirements. There are lots of embedded Ethernet boards that offer comparable performance/price ratios. To me, the Arduino seems slightly ahead in ease of use for beginners, but that's about it.
(Edit:) As competitors, I'd offer:
http://makezine.com/controller/
http://www.rabbit.com/products/
http://www.gumstix.com/store/catalog/index.php
http://www.embeddedarm.com/products/index.php
They're all harder to use than an Arduino, but I think they win on performance/price. And they're not that hard to use. The Make controller, in particular, is quite easy.