Because we're making ourselves invisible, it's that much more important to brand the services we provide. Most databases give institutions the option of customizing the OpenURL link in some useful way. If your library is still using a default "SFX" link, you're making two big mistakes:
- Washington State University found that nobody knows what "SFX" is, so the only people that found full-text by clicking on the "SFX" button were the people that click on everything. 
- People that click on a generic OpenURL, or a "full text" link provided by the database (vide PubMed), don't know that it works because the library paid lots of money for it.
We've customized our SFX links to clearly indicate that it's taking you to something that is available because of the University, and we've suppressed database-provided full-text links as much as possible, in order to reduce confusion on the part of the user, or so we thought. There's been a discussion on the web4lib mailing list over the last couple of days about the inconvenience to the user of having to click on every single SFX link to find out which articles were available online, with some people arguing that we don't want to encourage students to limit themselves to just what's online. I have swung around to the idea that the earlier we can present the user with a clear indication that the article is available right now, the better.
As a result of this change of mind, I'm starting to come around to registering MPOW for Google Scholar. Historically my concerns were both practical and philosophical: We dont' want to give Google our holdings because it's a workload (even if it's minimal) and because the whole point of OpenURL was that we didn't have to configure our holdings into the databases; that was the resolver's job. But the branding opportunity of making it clear in Google that the link works because of the university can only be a good thing for the library's visibility. Now, if only the branded"OpenURL" links that google generated were even close to useful, I'd be really happy.
 Cummings, Joel, and Ryan Johnson (2003). "The use and usability of SFX: context-sensitive reference linking", Library Hi Tech 21(1): 70-84 [link].