Well, I unzipped the Jet 4.0 SP8 for win2k3 and have confirmed that MSJETOLEDB40.DLL is NOT included in that distribution.
The version number (4.00.8227) registered on the server and in admin_info.asp is from the recent hotfix that MS put out. Sooooo... I still don't know why it's showing an OLEDB version number of 2.1 :(
Ok, now here goes something weird. At my home computer (XP Pro) I have the latest Jet service pack installed. Just tried the Jump To Last Post link and it worked as expected. However, OLEDB version is shown as 2.10 and provider version is 04.00.8227. It seems like the OLEDB version number really hasn't to do it the issue, has long has MDAC is 2.5 or higher. Unfortunately I have no explanation for what happens under my work computer (Win 2k), where I have a similar setting and the Jump To does not work.