Datagrid not automatically updating Datatable

Chet

I have a Datagrid that is bound to a Datatable at runtime. I allow the
user to select a number of rows using the mouse and then click a button
that says "check selected rows", which then cycles through the selected
rows and sets a check box field in the datagrid (and thus the databound
datatable) to checked/true. It works like this:

Dim cel As DataGridViewCheckBoxCell
For Each row In DataGrid.SelectedRows
cel = row.Cells(0)
cel.Value = True
Next

I then have a property / function I expose that should show me the
number of currently checked rows. It looks like this:

Public ReadOnly Property CheckedRowCount() As Integer
Get
Dim v As DataView = New DataView(DataTable)
v.RowFilter = "[Selected]=true"
Return v.Count
End Get
End Property

When I select a set of rows and then click the button that runs the
first set of code, the property represented by the second set is always
ONE LESS than the number of rows selected. It's like it's not
automatically marking the last databound datarow as updated... is there
a way to force this? I've tried programmatically changing the
CurrentCell property of the datagrid to another row, and this works, but
it doesn't look good at all, and the user loses the cells they selected.

Any ideas?

P.S. I have also tried updating the underlying datarow instead of the
cell value, as in:

For Each row In DataGrid.SelectedRows
dim dr as datarow= CType(row.DataBoundItem, DataRowView).Row
dr(0)=true
Next