We're looking into this right now for a similar sized client who needs to replace their SBS 2003 server. The way we look at it is, they need to have a server on-site anyways as they have several line of business applications that wouldn't work or aren't offered as Software-as-a-Service (SaaS).
Factoring that in, well, the (billable) time it would take to migrate all their mailboxes and data to a cloud solution (as I understand it each mail profile needs to get transferred twice: once to the cloud, once back down to create an offline .OST) plus the domain migration, troubleshooting, network changes (see below) etc. is likely pretty close to the same amount of time it would take to do a 2003 to 2008 migration, so there's really no savings in labor.
Now compare licensing of Windows SBS 2011 Standard vs. cloud-hybrid using Small Business Essentials plus Office365 or BPOS (and any/all the add-ins), and the savings are again, not much, certainly not much when you consider a year's worth of per-seat cloud fees.
There's also bandwidth/cap considerations to look at, plus upgrading network equipment for load balancing/redundancy (and likely getting a second Internet connection on a different carrier/network i.e. ADSL and Business cable Internet) that would be required if the business effectively shutdown due to any Internet outage when going with the cloud.
There's also x-factor stuff like Office macros/add-ins (you wouldn't believe how many offices out there have some "must have" Word or Excel macro) that might not work with Office365.
Basically, there's just a ton of variables here, and call me cautious, I just don't see most offices moving to a fully cloud-based offering right now. Hybrid/partial cloud services augmenting your current infrastructure? Absolutely, we're using Amazon EC2 for several clients for Web application hosting, Barracuda for off-site backup, and can definitely see the new Private Cloud offerings being a great fit for a disaster recovery failover. But completely moving to the cloud for everything? I just can't see not having some sort of server running locally and if that's the case, why not put SBS on it?