It looks like ESRI has consolidated the ArcGIS Server Development Blog on to their “blogs.esri.com” site. There still isn’t a aggregator for all their blogs like blogs.msdn.com or blogs.sun.com, but maybe in time.

The new URL is: http://blogs.esri.com/Dev/blogs/arcgisserver/ and http://blogs.esri.com/arcgisserver also redirects there. You’ll probably want to update your RSS feeds as well as I’m not sure if they’ll continue to work.