If the omapi_key is incorrect (I forced an incorrect key) Foreman's interface is not reporting the error.
In the proxy.log (in debug mode) I can see the error:

(...)
E, [2011-05-03T17:51:57.835019 #10819] ERROR -- : Omshell failed:
> > > obj: <null>
, > obj: host
, > obj: host
, hardware-address = 00:0c:29:38:71:62
, > can't open object: connection reset by peer
, obj: host
, hardware-address = 00:0c:29:38:71:62
, > can't destroy object: not connected
, obj: host
, hardware-address = 00:0c:29:38:71:62
, >
E, [2011-05-03T17:51:57.835393 #10819] ERROR -- : Failed to remove DHCP reservation for  => 192.168.115.156 / 00:0c:29:38:71:62: No response from DHCP