As a member of the Globus team at UChicago, I would be delighted if XSEDE chose to feature Globus prominently on its login buttons! And I agree with the rationale presented for why that would make sense.
However, what these buttons are offering is to allow users to login to an XSEDE application or service interface. The label "Globus Single Sign-on" sounds like an invitation to sign in to Globus, as opposed to an invitation to sign in to the application. As a user of the application, I don't want to sign in to Globus, I want to sign in to the application, so I'm unlikely to click that button.
If the powers that be in XSEDE agree that it's a good idea to indicate that Globus is being used, it might be clearer to label the button "Login with Globus" or "Login using Globus." That's actually more consistent with Web standards.
I'm not a user interface expert by any means, but I'll admit that even the "with Globus" wording above could be confusing for XSEDE users. Explanatory text next to the button would surely help me figure it out, but I'm not sure that's good design.
As a required reviewer for this design, I feel obligated to seek some assurance that others in the XSEDE project agree with this labeling and user interface design, and I'm reluctant to sign off on the design until I see that there's consensus for it, particularly from the folks who are primarily responsible for XSEDE's branding, "look & feel," user experience, user interfaces...