Quoting
http://testanything.org/wiki/index.php/TAP_version_13_specification#TODO_tests the spec says:

TODO tests

If the directive starts with # TODO, the test is counted as a todo test, and the text after TODO is the explanation.

not ok 13 # TODO bend space and time

Note that if the TODO has an explanation it must be separated from TODO by a space.

These tests represent a feature to be implemented or a bug to be fixed and act as something of an executable "things to do" list. They are not expected to succeed. Should a todo test point begin succeeding, the harness should report it as a bonus. This indicates that whatever you were supposed to do has been done and you should promote this to a normal test point.

AFAICT the TAP plugin doesn't seem to take care of the TODO directive accordingly though, therefore marking test runs as failures even though they should be fine.

Example:

http://michael-prokop.at/screeni/gkrellShoot_12-10-04_165641.png