I'm working with a client this week that needs to integrate a couple of different systems...
A mainframe application running under the Java 1.4.2 JVM with Websphere (as the available application server) - hitting against a DB2 database.
An ASP 3.0 web-based application running on a Microsoft Windows IIS Web Servere - hitting against a SQL Server 2000 database.
A few interesting problems involve how to manage units of work that span the two systems; working out a strategy to integrate the two databases - and ensuring that they do not get out of synch; and being sensitive to the need to achieve the best performance for the end-users of each system.
Some of the technologies / approaches to consider in solving these objecives will include: possible database replication to off-load potential reporting performance issues; Web Services to bridge the heterogenous application processing environments; and possibly using some type of Message Queue technologies to ensure delivery of events / transactions.