File extension does not matter. Reworking should be really straightforward. See FeedUpdateBroker handleResponse method:

var itemElements = xmlDoc.getElementsByTagName("item");
for...

This feed is atom. The widgets generated by Symbian WRT IDE and Aptana plugin only support RSS feeds. You would need to update parsing code in the FeedUpdateBroker.handleResponse

Atom feed is...

I was playing with Google Maps from WRT some time ago. I just put the code here - http://developer.symbian.org/wiki/index.php/Using_Google_Maps_with_WRTKit

It has some bugs and it requires WRTKit....

I've seen a bug on 5800 when the widget could've been "partially installed".
1. (Just a test) Try to change widget ID. You have the same problem I saw if you can deploy the widget with changed ID....

It should work there. There are two project templates (RSS and Flicr) that do AJAX calls to a different domain. You can create one of those and try if remote call works there. If not - you can open a...