I'm currently working on designing our BES 5.0 infrastructure. The plan is to upgrade to new BES hardware, setup a new database, and utilize the transporter tool to migrate the users. We're on Exchange 2007 with over 1500 users on 3 BES 4.1.6 servers. The BES servers are virtual on ESX 3.5. My initial thoughts on the architecture would look like the following:

Database:
BESDB01

3 BES pairs:
BES01A
BES01B
BES02A
BES02B
BES03A
BES03B

All services (Attachment, router, dispatcher, mds-cs, collaboration...) would be installed on each server. All the new servers will be Windows 2008 r2.

To support additional users, a 'scale-out' method would be used. As each BES pair reaches approximately 1000 users, a new BES pair would be added.

The BlackBerry Administration Service would be setup and installed on all of the BES0XA nodes and to support HA of this service, I'd utilize a hardware based network load balancer.

I'm considering log shipping or clustering for SQL but haven't decided just yet. The HA features offered by ESX might just be enough - I'm not sure I want to complicate this any further.

Several questions:
-How does this architecture look?
-I read that Windows 2008 and attachment services doesn't support .mp3's. Really - is this true?
-Are there some standard practices or recommendations made by RIM or other communities?
-Is there any reference architecture documentation out there?
-What did you do?

Thanks