I\'m going completely crazy:
Installed Hadoop/Hbase, all is running;
/opt/jdk1.6.0_24/bin/jps
23261 ThriftServer
22582 QuorumPeerMain
21969 NameNode
23
The problem was actually that (for some reason ... I don't really get it in detail) the firewall was blocking one of the ports required to talk to Zookeeper; from the command line it worked, from my app it didn't. However when I disabled the firewall all worked fine all of a sudden.
Thank you for your help!