It seems that some earlier distributions had distinguished the 2.0 Mono runtime from the 1.1 Mono runtime by having both nunit-console and nunit-console2 executables on the path, presumably for backwards compatibility.
However, the Mono distribution for Ubuntu 10.10+ currently uses the 2.0 Mono runtime for nunit-console executable, and nunit-console2 does not exist. Therefore, the current convention breaks the build when nunit-console2 cannot be found.
The contents of nunit-console are as follows:
$ which nunit-console
$ cat /usr/bin/nunit-console
exec /usr/bin/cli /usr/lib/nunit/nunit-console.exe "$@"
$ ls -l /usr/bin/cli
lrwxrwxrwx 1 root root 21 2010-02-22 00:03 /usr/bin/cli -> /etc/alternatives/cli
$ ls -l /etc/alternatives/cli
lrwxrwxrwx 1 root root 13 2010-02-22 00:03 /etc/alternatives/cli -> /usr/bin/mono
It looks as though an attempt was made to retain some degree of backwards compatibility for direct references to nunit-console2.exe, as shown by the contents of /usr/lib/nunit.
$ ls -l /usr/lib/nunit
lrwxrwxrwx 1 root root 17 2011-04-19 16:34 nunit-console2.exe -> nunit-console.exe
-rwxr-xr-x 1 root root 4096 2010-01-29 00:59 nunit-console.exe
-rw-r--r-- 1 root root 3736 2010-01-29 00:59 nunit-console.exe.config
-rw-r--r-- 1 root root 19456 2010-01-29 00:59 nunit-console-runner.dll