I can reproduce this problem with 7u6.
http://www.java.net/forum/topic/jdk/java-se-snapshots-project-feedback/os-x-jdk-7u6-will-not-run-signed-jnlp-apps

java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: java.security.InvalidKeyException: Wrong key usage
at com.sun.deploy.security.TrustDecider.doCheckRevocationStatus(Unknown Source)
at com.sun.deploy.security.TrustDecider.validateChain(Unknown Source)
at com.sun.deploy.security.TrustDecider.isAllPermissionGranted(Unknown Source)
at com.sun.javaws.security.AppPolicy.grantUnrestrictedAccess(Unknown Source)
at com.sun.javaws.security.JNLPSignedResourcesHelper.checkSignedResourcesHelper(Unknown Source)
at com.sun.javaws.security.JNLPSignedResourcesHelper.checkSignedResources(Unknown Source)
at com.sun.javaws.Launcher.prepareResources(Unknown Source)
at com.sun.javaws.Launcher.prepareAllResources(Unknown Source)
at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
at com.sun.javaws.Launcher.launch(Unknown Source)
at com.sun.javaws.Main.launchApp(Unknown Source)
at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
at com.sun.javaws.Main.access$000(Unknown Source)
at com.sun.javaws.Main$1.run(Unknown Source)
at java.lang.Thread.run(Thread.java:722)
Caused by: java.security.cert.CertPathValidatorException: java.security.InvalidKeyException: Wrong key usage
at sun.security.provider.certpath.OCSPResponse.verifyResponse(OCSPResponse.java:541)
at sun.security.provider.certpath.OCSPResponse.<init>(OCSPResponse.java:494)
at sun.security.provider.certpath.OCSP.check(OCSP.java:261)
at sun.security.provider.certpath.OCSP.check(OCSP.java:165)
at sun.security.provider.certpath.OCSP.check(OCSP.java:130)
at com.sun.deploy.security.TrustDecider.doOCSPEEValidation(Unknown Source)
... 16 more
Caused by: java.security.InvalidKeyException: Wrong key usage
at java.security.Signature.initVerify(Signature.java:490)
at sun.security.provider.certpath.OCSPResponse.verifyResponse(OCSPResponse.java:524)
... 21 more