Following upgrade to 1.5, github pull request builder couldn't connect to github because the default setting for the github api url (which is also specified by the config validator) is api.github.com. This causes the following error:

Feb 21, 2013 5:59:40 PM org.jenkinsci.plugins.ghprb.GhprbRepo connect
SEVERE: Can't connect to api.github.com using oauth
java.net.MalformedURLException: no protocol: api.github.com/user?access_token=XXXXXXXXXXXXXXXXXXXXXXXXXX
at java.net.URL.<init>(URL.java:585)
at java.net.URL.<init>(URL.java:482)
at java.net.URL.<init>(URL.java:431)
at org.kohsuke.github.GitHub.getApiURL(GitHub.java:175)
at org.kohsuke.github.Requester._to(Requester.java:159)
at org.kohsuke.github.Requester.to(Requester.java:139)
at org.kohsuke.github.GitHub.getMyself(GitHub.java:200)
at org.kohsuke.github.GitHub.<init>(GitHub.java:102)
at org.kohsuke.github.GitHub.connectUsingOAuth(GitHub.java:149)
at org.jenkinsci.plugins.ghprb.GhprbRepo.connect(GhprbRepo.java:61)
at org.jenkinsci.plugins.ghprb.GhprbRepo.checkState(GhprbRepo.java:73)
at org.jenkinsci.plugins.ghprb.GhprbRepo.check(GhprbRepo.java:91)
at org.jenkinsci.plugins.ghprb.GhprbTrigger.run(GhprbTrigger.java:132)
at hudson.triggers.Trigger.checkTriggers(Trigger.java:259)
at hudson.triggers.Trigger$Cron.doRun(Trigger.java:207)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)

Changing the config value to https://api.github.com worked.