single signon / openbar type of app

I recently stumbled onto the OpenBar (http://www.37signals.com/
openbar) feature within the different 37signals apps and would like to
do something like this for the internal apps I am working on right
now.

I'm almost finished with a little custom openid server app that
manages profiles for employees as well as being able to define what
sites users have access to. As of right now, they can sign into that
server and see their apps and just click on a link from there to sign
in, but having something always on each site with a list of available
apps would be even better.

I'm just wondering what would be the best way to do something like
that. The only way I can think of is building a web service on top of
my openid server that each app can request the other apps for the
currently logged in user, but as I haven't worked with web services
much, I'm not sure about the most appropriate way of getting/storing
the data. I'm guessing I would want to cache the request until the
user logs out so that I wouldn't be sending a request to my openid app
each time a page is loaded, but I'm not sure.

I would love to hear any suggestions from anyone!
Thanks.