Currently, this is a limitation in the actual storage service. The LINQ provider eventually boils down to a URI with some syntax that is or isn't supported on the service head. It turns out that certain operations that are supported in relational sources are not supported in Windows Azure tables. Your specific question would entail a full-text search to be performed on an arbitrary column in Windows Azure. This is not currently supported (there is no LIKE operation). This limit might be removed over time as more features are added to the service.
If you really need full text search capability over your data - take a look at the Lucene implementation here: