This update from the StreamInsight Team at Microsoft:
The premium version of StreamInsight is recommended when event rates exceed 5000 events per second or latency requirements are less than 5 seconds. For event rates less than 5000 events per second and/or latency requirements in excess of 5 seconds then we recommend the standard version of StreamInsight. The premium version is included with SQL Server 2008 R2 Datacenter, while the standard version comes with SQL Server 2008 R2 Standard and Enterprise.
As you might already know, SQL Server 2008 R2 Datacenter version is licensed per processor only (not per server +CAL) and the suggested price is $57,498 per processor (which is about twice the price of the Enterprise Edition).