The November CTP for SQL Server 2008 introduced our new Lookup Transform. 2005 packages with lookup components will upgrade automatically when opened/run in 2008, and should function the same as they did in 2005 with one minor caveat.
In certain scenarios, dataflows containing lookups with matching SQL queries would implicitly share a cache. As this implicit sharing was somewhat error prone, it has been removed in 2008. To share a lookup cache, you can now use the new cache connection manager.
For more info about the 2008 lookup changes, you can take a look at Jamie Thomson's write up.