Thanks, Kohsuke! This was exactly the OpenId4Java subclass I was hoping would be possible! (Shout out to @autojack for channeling me IRL yesterday on IRC.)

I got the hint for this approach from a Ruby gem published by Living Social. The minimum OpenID workaround is encapsulated here:
https://github.com/sodabrew/rails-googleapps-auth/blob/master/lib/googleapps_auth.rb#L151-168

Damn it Kohsuke,
you resolved it just before I got the chance to make an offer for this issue at FreedomSponsors.

I'm taking the liberty of reopening it ONLY to draw your attention for it, hope you don't mind.
Here's the offer --> http://www.freedomsponsors.com/core/offer/2/make-openid-work-with-google-apps-accounts
Please go there and mark it as resolved and I will gladly pay up - as soon as the fix is available in a release version of Jenkins - (or you can wait a couple of days and maybe someone else will join me as a sponsor, it's up to you, really)

Everyone else, just to explain what this is about:
I'm a big fan of Jenkins (and Kohsuke's), and I created FreedomSponsors, specifically to make him rich (and maybe other people like him who also deserve it)

And this is the very issue that I was looking at, when the whole idea hit me.

That was like, 7 weeks ago. Now it has been up and running for a couple of days.

Yes, I have every intention of creating a big positive impact on open source projects like Jenkins. So please feel free to join and spread the word about it.

Cheers
Tony Lâmpada
FreedomSponsors - http://www.freedomsponsors.com
Software engineer and CEO, I guess

Thank you! I just updated that offer and also marking this as closed.

No, thank you!
I'll check it out

It's actually not working for me yet.

The weirdest thing happens: it works for everyone else in the company except for me
(And I already checked with HR: I have NOT been fired... hehe)

So, when I come back from Google Apps' login page, I get stuck on a HTTP redirect infinite loop on "/jenkins/securityRealm/finishLogin?openid.ns=http%3A%2F%2Fspecs.openid.net%2Fauth (....)"

There seems to be something worng with my account, specifically, that's triggering this srange behaviour.

I have jenkins running inside tomcat, on port 8181, and I'm accessing it directly (without apache).
Also, this is a brand new jenkins installation (meaning: there was no .jenkins folder before I started tomcat) with OpenId plugin 1.5 newly installed.

I'm attaching a piece of the log from catalina.out (I changed the domain name though...)

Should I open a bug for this?

Cheers