It appears that Interface21 is now SpringSource. You can read all about the name change in Rod's latest blog entry but it seems to be the case that they wanted the company name to reflect the product base. I think that Neelan's latest blog entry says it all:
...the Spring Framework is spreading like wildfire throughout the application development and runtime universe—most as a response to developers frustrated with the complexities of Java EE
I saw this trend starting back around 2002 when we formed Apache Geronimo and even moreso in 2003 I was still doing some consulting work with application servers (and, this was a major driver for Geronimo's very modular architecture allowing you to roll your own app server assembly). Also, in 2003 I started using the Spring Framework and really began moving beyond application servers. Now, everyone from developers to architects are finally realizing that a big app server is not necessary on every node, especially when you have the Spring Framework.
Long live Spring ;-).