I recently developed an SSIS package on my dev machine using the 2008 version. Later I foud the customer had Sql server 2005 and doesn\'t plan to upgrade anytime soon.
In addition to Asgeir:
The correct ClassID for OLE DB sources in 2008 version is {BCEFE59B-6819-47F7-A125-63753B33ABB7}. This has to be changed to {2C0A8BE5-1EDC-4353-A0EF-B778599C65A0} to be used in 2005.
For lookup tasks change {27648839-180F-45E6-838D-AFF53DF682D2} into {0FB4AABB-C027-4440-809A-1198049BF117}. Furthermore you have to change the term 'version="5"' into 'version="4"' in the line below the ClassID.
For conditional split task change {3AE878C6-0D6C-4F48-8128-40E00E9C1B7D} into {53A228EE-EBFA-48D6-A1AC-5269E5824A2C}.
These changes in addition to the ones already posted should do the trick for most users or at least make it possible to rebuild the package in 2005 version with less effort.