A little birdie once told me that Cloud Based Detection for SEP is in the offing, but not just yet. There is extensive research going on w.r.t. this, and they'd come to market with an offering once it has been very very rigorously tested. The initial concept dates to around a year or so back, by taking on cue Salesforce.com as a great exapmple for Cloud based services.
ANd trust me, when SYmantec goes to market with such a Cloud based detection in the future, most of the other vendors would then start discovering that they've let some or the other loopholes in their offerings.
I'd really jump at the prospect of being amongst the first to test this, as my work entails working extensively on Cloud Computing environments.
All I can say is, wait and watch.....