I was just researching a question on SQL Server replication, whether SQL Server 2000 can participate with SQL Server 2008 in a replication strategy. The answer is yes:
SQL Server 2000 and SQL Server 2005 can both participate in replication topologies with SQL Server 2008. For SQL Server 2000 the minimum version is Service Pack 3. For SQL Server 2005 the minimum version is Service Pack 2.
Ref: Using Multiple Versions of SQL Server in a Replication Topology
When you replicate between or among different versions of SQL Server, you are usually limited to the functionality of the earliest version used. For example, if you upgrade a Distributor to an instance of SQL Server 2008, but you have a Publisher that is running an instance of SQL Server 2005 and a Subscriber that is running an instance of SQL Server 2000, you are limited to the general functionality and replication functionality of SQL Server 2000.
Because the SQL Server on-disk storage format is the same in the 64-bit and 32-bit environments, a replication topology can combine server instances that run in a 32-bit environment and server instances that run in a 64-bit environment.
There are some breaking changes in the replication agent security model detailed here: Breaking Changes in SQL Server Replication