For future reference:

• go to http://dist.opensimulator.org/ and get the latest .tgz
• aptitude install:
• mono
• libmono-corlib2.0-cil
• libmono-sqlite2.0-cil
• libmono-system-web2.0-cil
• libmono-microsoft8.0-cil
• libmono-system-runtime2.0-cil

• unpack the .tgz, cd opensim and nant

Now you're ready to go :-)

(More extensive info at http://opensimulator.org/wiki/Main_Page)