Actually Approval is a cut down version of the full 5.0x product, this is given away by the fact that the 5.05 update is the same file for both applications!. You can't create pdfs, modify form fields etc, but you can digitally sign, save, apply security etc.

Approval IS still available on the Adobe web site,

<http://www.adobe.com/store/products/master.jhtml?id=catAcrobatApproval>

but you can't get licencing through an Adobe distributor anymore :(

(well not the one we used in the UK anyway)