Chris,Below is a VBA script I knocked up to test the whole in_memory workspace thing. I know it's not .Net but this should pretty much port into your code and gives you an idea of what worked for me?DuncanPublic Sub InMemoryExample()
 ' Description: Sample code showing how an in memory table can be created
 
 ' Create Workspacefactory
 Dim pWorkSpaceFactory As IWorkspaceFactory2
 Set pWorkSpaceFactory = New InMemoryWorkspaceFactory
 
 ' Create Workspace
 Dim pWorkspaceName As IWorkspaceName2
 Set pWorkspaceName = pWorkSpaceFactory.Create("", "MyInMemoryWorkspace", Nothing, 0)
 
 ' Get a handle on Name
 Dim pName As IName
 Set pName = pWorkspaceName
 
 ' Get a handle on FeatureWorkspace
 Dim pFeatureWorkspace As IFeatureWorkspace
 Set pFeatureWorkspace = pName.Open
 
 ' Now create a fields object and add some fields to it
 
 ' Create Fields object and set the number of fields
 Dim pFields As IFields
 Set pFields = New Fields
 Dim pFieldsEdit As IFieldsEdit
 Set pFieldsEdit = pFields
 pFieldsEdit.FieldCount = 2
 
 ' Create objectID field
 Dim pField As IField
 Set pField = New Field
 Dim pFieldEdit As IFieldEdit
 Set pFieldEdit = pField
 With pFieldEdit
 .Name = "OBJECTID"
 .Type = esriFieldTypeOID
 End With
 Set pFieldsEdit.Field(0) = pField
 
 ' Create Type field
 Set pField = New Field
 Set pFieldEdit = pField
 With pFieldEdit
 .Editable = True
 .Name = "VegType"
 .Type = esriFieldTypeString
 .Length = 4
 .AliasName = "Vegetation Type Code"
 End With
 Set pFieldsEdit.Field(1) = pField
 
 ' Create Table
 Dim pTable As ITable
 Set pTable = pFeatureWorkspace.CreateTable("MyInMemTable", pFields, Nothing, Nothing, "")
 
 ' Create a sample row and add data
 Dim pRow As IRow
 Set pRow = pTable.CreateRow
 With pRow
 .Value(1) = "WOOD"
 .Store
 End With
 
 ' Add table to map document
 Dim pMXD As IMxDocument
 Set pMXD = ThisDocument
 Dim pMap As IMap
 Set pMap = pMXD.FocusMap
 Dim pStandAloneTableCollection As IStandaloneTableCollection
 Set pStandAloneTableCollection = pMap
 Dim pStandAloneTable As IStandaloneTable
 Set pStandAloneTable = New StandaloneTable
 Set pStandAloneTable.Table = pTable
 pStandAloneTableCollection.AddStandaloneTable pStandAloneTable
 pMXD.UpdateContents
End Sub