We came across this recently when setting up a shared mailbox on a number of fruity devices (ok, iPads) for one of our schools. For some strange reason the account setup didn't fail but devices 11 onwards weren't able to sync. Luckily, Exchange does send you an email about this when you try to add a device so you're not left in the dark for long.
The reason for the failure is because Exchange 2010 SP1 by default restricts mailboxes to 10 ActiveSync devices - which it accomplishes by way of a 'throttling policy'. Fortunately, this policy is configurable and you can adjust it upwards to suit your needs. You can also create & assign different throttling policies to different mailboxes if that is what you need.
The full reference is available at technet and there is a blog post about the association aspect at http://blogs.technet.com/b/exchange/archive/2010/08/02/3410563.aspx.
To get straight to the good stuff you only need two cmdlets:
get-mailboxthrottlingpolicy
(make a note of the -identity)
set-mailboxthrottlingpolicy -identity <insert identity from previous step> -easmaxdevices xx
where xx is the number of devices you want to allow.
Simples!