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.add(networkType,strUrl,objContext,p_strPostArgs,p_objHeaders,p_flags)
batchRequest.execute(p_fnCallback);
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!

[update: More info at  http://microsoftgadgets.com/ and http://spaces.msn.com/members/siteexperts/]