A “senior advisors” threat is not unusual, but it’s certainly a level below a presidential veto threat and suggests they want some wiggle room. In the Bush administration (in which I served as a quite junior, not senior, advisor) moving from a senior advisors veto threat to a presidential veto threat was a very significant step, implying no room for maneuver. It’s not clear if the Obama White House treats the difference the same way, but presumably it’s along the same lines.
More interesting than that, though, is the fact that the Statement of Administration Policy that includes the threat contains no substantive critique of the Boehner bill. Generally, statements like this (particularly when they contain veto threats) include several paragraphs (sometimes several pages) of argument about what’s wrong with the bill. The administration’s SAP about the Cut, Cap, and Balance Act earlier this month, for instance, listed numerous concerns about the bill, and ended with a presidential veto threat. Offering no arguments about the Boehner bill suggests the White House wants to leave open the possibility of signing this bill or one much like it, by avoiding committing to critiques of certain parts of it that would then make it difficult to sign legislation that included those parts.
In any case, I think there is zero chance that the president would veto a debt-ceiling increase that passed both houses of Congress. The politics of that would be horrendous for him, and there is no way he would do it. It’s an empty threat.