Google AppEngine is a fun environment to develop for.  AppScale is an open source implentation of Google AppEngine that can be used on your own equipment.  This new software project works on Amazon’s  AWS and EC2 platforms and the open source version of that in Eucalyptus.

Cloud based architectures I believe will be more mainstream and used for development even if on self-hosted environments because the data architecture is setup for maximum horizontal scalability from the beginning.  BigTable, SimpleDB, HBase, even couchDb type databases will become more prevalent as we move into terabytes and terabytes of data.  Data is growing at an alarming rate and this is a natural software evolution to address that.