I discovered it by accident: I was logged into Google+ with my Google Account and into my Google Apps account as secondary, with multiple sign-on. For some reason Google decided to log me off all my Google accounts. At that point, I made the initial login with my Google Apps account and secondary with my Google Account. I was unaware that Google+ was open on another tab. I tried to reshare something publicly, and was confused that it was saying Paralaus where it should be saying Public. Picking that options told me that only "People on Paralaus can find and view" my post. It was then that I realized I was able to use Google+ with my Google Apps account in a hybrid mode where I was not fully logged in but some elements were available; almost as a preview.
Right now, there's no support for Buzz or Profiles in Google Apps, but that should change in the near future. Google's John Costigan confirmed this: "We're actively working on making Profiles (and Google+) available for Google Apps - it should be available in the coming months."
Google+ is a very important project for Google, so it will improve rapidly and it will no longer require invitations. Some speculate that Google+ will be available for everyone at the end of the month.
{ Thanks, Ufuk. }