Beth Israel Hospital in Boston has an authorized signature process for
every expenditure.  An administrator within each division has signature
approval up to $250. with a co-sign for $250 and above.  We have recently
given PIs signature authority for purchase requisitions for laboratory
supplies and have had little difficulty to date.  We do carefully line
audit all grants so that the PI's do not get "out of control".
Fortunately, the PIs (3) in the division are very responsible and they
have passed this beta test.  Interestingly enough, not many PIs have
asked for the privilege of signature authority (probably because they
are unaware that this is an option).  I certainly like the control that
this brings for accountability, but not the expense of the process.