I have just updated my CSS 3 Selectors tests to reflect a decision made by the CSS WG during its 2008 San Diego face-to-face meeting :

Attribute Selectors
-------------------

RESOLVED: Copy attribute selector wording from Selectors into CSS2.1 to
resolve incompatibility.

RESOLVED: *= ~= ^= $= all accept the empty string, but match nothing.
Note that this overrides a previous resolution which stated
that such attribute selectors are invalid.
RATIONALE: These selectors are meaningless. This resolution matches
existing implementations for ~=, which was introduced in
CSS2.1, and makes *=, ^=, and $=, which were introduced in
Level 3,
[note, see also http://lists.w3.org/Archives/Public/www-style/2008Apr/0021.html
which was posted in response to this]

Firefox 3.1a1pre still has two bugs related to [align^=""] and [align$=""] incorrectly matching the attribute align="center".