The person who, IMHO, will really be hurt by this is Rudy. I've long felt he has a good chance for the 2008 GOP nomination, and that he could become acceptable to social conservatives and evangelicals in the GOP because of his strong stance on national security issues, but if Rudy is forced to take a position on this issue, and agrees with Bloomberg..then he's toast...so there may be a political implication at play here..
I think Guiliani is already on record for civil unions which are federally recognized as marriage in all but the "M" word.
The ONLY way Guiliani can recover would be to announce support for the Federal Marriage Amendment (which as a lawyer, he KNOWS is the only way to make normal marriage the law of the land safe from judges) AND reounce the sham civil unions.
This he also knows as a lawyer since cohabitation agreements, advance directives take care of the civil union issues.