@Brian:

Even if it were against the CC license (it's not, as I'll explain below), but it is an unenforceable restriction.

Even for someone with as restrictive of a view of theme GPL applicability as me, I would concede that the part of the theme in question (the footer) is clearly a WordPress-derivative work; therefore, the copyright in question belongs to WordPress and the controlling license is the GPL.

Also, the CC license's attribution clause only applies to re-distribution of the work.

Further, I don't think that the Creative Commons "Attribution" clause applies to requiring users of a work to include a mandatory, third-party script call. In fact, here is the actual wording of the license, regarding attribution (added emphasis, mine):

If You Distribute, or Publicly Perform the Work or any Adaptations or Collections, You must, unless a request has been made pursuant to Section 4(a), keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or if the Original Author and/or Licensor designate another party or parties (e.g., a sponsor institute, publishing entity, journal) for attribution ("Attribution Parties") in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; (ii) the title of the Work if supplied; (iii) to the extent reasonably practicable, the URI, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and, (iv) consistent with Section 3(b), in the case of an Adaptation, a credit identifying the use of the Work in the Adaptation (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). The credit required by this Section 4(c) may be implemented in any reasonable manner; provided, however, that in the case of a Adaptation or Collection, at a minimum such credit will appear, if a credit for all contributing authors of the Adaptation or Collection appears, then as part of these credits and in a manner at least as prominent as the credits for the other contributing authors. For the avoidance of doubt, You may only use the credit required by this Section for the purpose of attribution in the manner set out above and, by exercising Your rights under this License, You may not implicitly or explicitly assert or imply any connection with, sponsorship or endorsement by the Original Author, Licensor and/or Attribution Parties, as appropriate, of You or Your use of the Work, without the separate, express prior written permission of the Original Author, Licensor and/or Attribution Parties.

Your only control over attribution is your right to have:

The license remain intact
Your name, in the manner you specify, appear in the copyright notice
The title of the work, in the manner you specify, appear in the copyright notice
A URI (which must reference the copyright information for the work) appear in the copyright notice
(In the case of an adaptation) credit of your original work remain in the copyright notice

Technically speaking, one could put all this information in the style.css header information, and be fully compliant with the license - with or without inclusion of/modification to the footer.