There is a version in info file '7.x-2.0-alpha2+2-dev' in last dev version.
Which is incompatible with dependency format - http://drupal.org/node/542202 (see for example http://drupal.org/project/twitter_bootstrap_ui).
I assume it should be named like '7.x-2.x'.
The drupal.org packaging script injects this version string. As you can see, it is not contained in the source code:http://drupalcode.org/project/libraries.git/blob/HEAD:/libraries.info
In any case, the claim that this version string would be incompatible with dependency versions is not true. Drupal core (actually PHP) handles the versions correctly.
You likely want:
dependencies = libraries (>=2, <3)
The documentation page http://drupal.org/node/542202 might be outdated.
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