Cisco equipment is WECA certified to be interoperable with any other vendors bearing the WIFI mark.
More info here; http://www.wirelessethernet.org/index.html
WECA only test for 802.11 compliance, which only requires 40bit WEP, so that is all the WECA/WIFI certification guarantees interoperability for.
Many vendors (including Cisco) implement 128bit WEP. If equipment interoperates at 40bit WEP, chances are, it will also interoperate at 128bit (if implemented), however there is no formal interoperability testing done for 128bit.