Can Exchange 2010 high availability be achieved with only 2 servers?

Yes. It can be done quite well. Do a basic Exchange install on each server (Mailbox, CAS, and Hub). Setup a CAS Array and assign it to the mailbox

New-ClientAccessArray –Name “CAS Array” –Fqdn “exchange.domain.com” –Site “Default-First-Site-Name”
Set-MailboxDatabase DatabaseName -RpcClientAccessServer “exchange.domain.com”

Setup a DNS record pointing exchange.domain.com to your hardware load balancer. Then setup your DAG, create mailbox copies, and you're all done.

You also might want to check out yesterday's Exchange team blog. It doesn't talk about a single server but does go over your options for load balancing your CAS servers with ISA (now called TMG) and HLBs. http://msexchangeteam.com/archive/2010/07/20/455575.aspx

http://rebootuser.com/2011/01/05/exchange-2010-configuring-dag-on-a-2-node-cluster-each-running-cas-ht-mb-roles/

By this method you can configure 2 servers with MB/HT/CAS and DAG with High Availability. If one server is out of service owa and outlook will work fine. Please be note that when we create DAG, windows will automatically create a cluster IP. That you can use as "cas array" dns entry.