what's your definition of 'stable'?
The current alpha code we are using internally has no problems with it's uptime, it hasn't caused any data loss in the server-side mailbox structures (and the way it works, it actually can't do that, really) and the issues I've seen have been around push email not reliably working in the case of network disconnection recovery, some email messages with complex nested content being not readable on the device and some cases of contact record duplication on change.
I'd personally call that stable - as I said, I'm using it for my day-2-day mobile email and my volume is pretty high.
There will be known limitations in Beta 1 that will be documented in the release notes, most notably that we haven't enabled full attachment support, calendar scheduling and device management functionality yet as some parts there need further work. That's why it's considered a Beta, not a release candidate.
Would I rollout a Beta version in production? Well, as I said, we internally do. For customers, I'd say it depends. Would I put my technology-phobic CEO on it that really relies on his wireless email to work 100%, 24x7? Probably not. I guess you see my point.
As for deployment and Scalix server compatibility... Beta 1 will need to be installed on a separate machine and will work against a Scalix Server 11.4.1 or higher. Future Betas may and the final product will have various deployment options, including the separate server deployment or actually being installed on the Scalix Server. In the latter case, the Scalix Server will have to be upgraded to 11.4.2 or maybe even 11.4.3 as they become available (don't ask for dates here, please, this is work in progress, although 11.4.2 patch release is only a couple weeks away) - during ActiveSync development we've found a some things on the server side that we wanted/had to improve to ensure best possible compatibility, performance and user experience.
Obviously the main reason why we've moved out release dates for ActiveSync as compared to our original plan is that we'll only ship this once it has reached the quality and maturity level our customers expect from us. Turns out that even though we've licensed the protocol specifications from Microsoft, some pretty deep reasearch was still required as sometimes not everything that's written on paper is as accurate as one would hope to be. Also, the various device vendors - most notably Apple with the iPhone - actually use the ActiveSync protocol in quite different ways and one has to accomodate for all of them.
I'm using this interchangeably with iPhone, Windows Mobile and Symbian-based devices, btw. Haven't tried the Versamail/Palm yet, but assume it's not really a huge factor in the market anymore and I believe our QA team is playing with that as well.
Florian