Sigh. It would *appear* that the Microsoft Pos .Net library and the .Net 4.0 framework are not 100% compatible. This Pos .Net Forum post explains the problem, which is basically that the Pos .Net binaries include CAS policies (which they should have in previous versions) and .Net 4 has dropped CAS support in many cases. As a result .Net 4 now generates a warning/error unless the application configuration file is modified to specifically re-enable CAS.
Hopefully Microsoft will provide a fix for Pos .Net soon... but perhaps more of a concern is what other libraries, be they 1st, 2nd or 3rd party, are now broken ? You're own libraries you can update easily enough (probably), but if this becomes a prevalent problem with other .Net dependencies it could be a real headache !