It looks like it is possible to create your own start.com site, develop "gadgets" for it, etc. Go directly here: http://start.com/developer/
Here is also some documentation on the new Atlas runtime: http://start.com/developer/atlasruntime.aspx
While you are there, check out the infrastructure for Atlas bindings. But what I like most is the new seamless AJAX-based programming model - for example here is how you can create a batch of XMLGet requests. No more fiddling with arrays of XMLHTTP objects, checking their state, etc.
batchRequest = Web.Network.createBatch([priority[,objContext]])
// Add as many requests
batchRequest.abort() // Abort the requests
Lots of fun. I remember the days where I browsed the web in Mosaic for the first time. I thought that loading the images in background was cool... now I can do easily the same with some easy DHTML code!