For fun I've been trying to get Cayenne running on Google App Engine (GAE) using the in-memory HSQLDB database. GAE has quite a number of restrictions such as disallowing creation of new threads. More importantly GAE isn't backed by a RDBMS and so JDBC isn't supported, however in-memory databases could work (in theory at least).
H2Database: http://www.h2database.com/ works on GAE/J (in memory). See http://www.h2database.com/html/changelog.html since "Version 1.1.111 (2009-04-10)"
Adrian.
