One additional comment: I also tested by setting port to 5223 with the same result.

INFO: Completed initialization
Feb 15, 2011 2:36:02 PM hudson.TcpSlaveAgentListener <init>
INFO: JNLP slave agent listener started on TCP port 58299
Feb 15, 2011 2:36:02 PM javax.jmdns.impl.HostInfo newHostInfo
WARNING: Could not find any address beside the loopback.
Feb 15, 2011 2:36:32 PM hudson.plugins.im.IMConnectionProvider$ConnectorRunnable run
INFO: Trying to reconnect
java.io.EOFException: input contained no data
<-->at org.xmlpull.mxp1.MXParser.fillBuf(MXParser.java:3003)
<-->at org.xmlpull.mxp1.MXParser.more(MXParser.java:3046)
<-->at org.xmlpull.mxp1.MXParser.parseProlog(MXParser.java:1410)
<-->at org.xmlpull.mxp1.MXParser.nextImpl(MXParser.java:1395)
<-->at org.xmlpull.mxp1.MXParser.next(MXParser.java:1093)
<-->at org.jivesoftware.smack.PacketReader.parsePackets(PacketReader.java:325)
<-->at org.jivesoftware.smack.PacketReader.access$000(PacketReader.java:43)
<-->at org.jivesoftware.smack.PacketReader$1.run(PacketReader.java:70)
Feb 15, 2011 2:37:35 PM hudson.plugins.jabber.im.transport.JabberIMConnection retryConnectionWithLegacySSL
INFO: Retrying connection with legacy SSL
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException
<-->at sun.security.ssl.Alerts.getSSLException(Alerts.java:192)
<-->at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1665)
<-->at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:258)
<-->at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:252)
<-->at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1165)
<-->at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:154)
<-->at sun.security.ssl.Handshaker.processLoop(Handshaker.java:610)
<-->at sun.security.ssl.Handshaker.process_record(Handshaker.java:546)
<-->at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:913)
<-->at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1158)
<-->at sun.security.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:652)
<-->at sun.security.ssl.AppOutputStream.write(AppOutputStream.java:78)
<-->at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:220)
<-->at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:290)
<-->at sun.nio.cs.StreamEncoder.implFlush(StreamEncoder.java:294)
<-->at sun.nio.cs.StreamEncoder.flush(StreamEncoder.java:140)
<-->at java.io.OutputStreamWriter.flush(OutputStreamWriter.java:229)
<-->at java.io.BufferedWriter.flush(BufferedWriter.java:253)
<-->at java.io.FilterWriter.flush(FilterWriter.java:100)
<-->at org.jivesoftware.smack.PacketWriter.openStream(PacketWriter.java:258)
<-->at org.jivesoftware.smack.PacketWriter.writePackets(PacketWriter.java:186)
<-->at org.jivesoftware.smack.PacketWriter.access$000(PacketWriter.java:40)
<-->at org.jivesoftware.smack.PacketWriter$1.run(PacketWriter.java:76)
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid cer
<-->at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:324)
<-->at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:224)
<-->at sun.security.validator.Validator.validate(Validator.java:235)
<-->at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:147)
<-->at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:230)
<-->at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:270)
<-->at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1144)
<-->... 18 more
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
<-->at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:197)
<-->at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:255)
<-->at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:319)
<-->... 24 more