First up, I've used the J2EE CTS - the notion of "BEA developers testing IBM's products" is broken. Every vendor must develop a porting layer to hook their product into the CTS, this porting layer generally contains internal API knowledge. Vendors are not required to make this layer public (and I'm certain they'd be unwilling to do so).
Secondly, the CTS only tests behavior and API conformance, it is not an exhaustive system test suite. Yes, there aree thousands of tests, but it's no silver bullet for J2EE testing.
Third, involvement with CTS generally needs backup from Sun's licensee engineering organization, this service costs money for Sun to provide. If OpenSource groups want to be involved in J2EE confirmance they can pay for the service.
Fourth, I've meet the Sun J2EE and CTS folks, I believe that Karen Tegan's response is being misinterpreted - "having a strong brand and compatibility standards are important ... The J2EE Compatible brand has achieved siginificant momentum overt the past two years, and we want to make sure that any open source efforts don't impact the viability" - my understanding of this statement is that it is not an attempt to exclude OpenSource projects, on the contrary, it is Sun expressing the hope that OpenSource efforts do not weaken the "J2EE Compatible brand" by claiming conformance to J2EE when they may not be (conformance is decided by sucessfully passing the complete J2EE CTS).