We've fixed the bug introduced in 0.5.5. that several people experienced related to sending to wrong contacts with a total overhaul of the "auto-completer" used in email to/from/cc/etc fields. The bug was the result of a small "fix" that didn't get tested well since we rushed 0.5.5 out to simplify validation of founder IDs. I want to keep the focus on getting Founder IDs registered until after the 28th, so we're holding off a new binary release with the completer fix until then. My current plan is to release a new binary release on Monday.
If you own a Founder ID, please try to register before the 28th deadline. However, there's no need to panic about the deadline, as we're going to "reserve" any unregistered Founder IDs, so you won't lose it if you miss the deadline.
Also, please note that Founder IDs can be registered and regenerated across OS platforms. In other words, you can create a profile with an identity on one computer (e.g. Windows), then later create the same profile with the same identity and key on another computer running a different OS (e.g. Mac). The only requirement is that you remember the information you entered when you first created the profile (First/Middle/Last Name, brainwallet key, and password) including the capitalization of the words you enter (this is really important for everyone to remember).
So if you're a Mac user who doesn't want to compile from source, you can register your Founder ID on a Windows or Linux machine for now, assuming you have access to one.