Windows Azure Diagnostics stores data in Azure Table. Unfortunately, Azure Table only supports a single index, which means queries that don't use the index are dog slow. In azure terminology, that index is the partition key, and Windows Azure Diagnostics sets the partition key on your behalf using the following formula: 0 + DateTime.Ticks
You can generate the Partition key query in powershell for all logs after 7/15 by doing:
PS C:\> "PartionKey gt '0$((Get-Date '2011/7/15').Ticks)'" PartionKey gt '0634462848000000000'
For the curious, gt is the greater than operator in Azure Table, which is based on ADO.Net DataServices, which is now called WCF Data Services, which is OData.