For the time being the following workaround can be used:

CSS:

    <style>
        .RadGrid_Default .RadButton_Simple input {
            border: 0 none;
        }
    </style>

ASPX:

        <telerik:RadGrid ID="RadGrid1" runat="server">
            <MasterTableView>
                <Columns>
                    <telerik:GridTemplateColumn UniqueName="EditColumn" AllowFiltering="false">
                        <HeaderStyle Width="70px" />
                        <ItemTemplate>
                            <telerik:RadButton ID="rbEdit" runat="server" Text="Select" Skin="Simple"
                                AutoPostBack="false">
                            </telerik:RadButton>
                        </ItemTemplate>
                    </telerik:GridTemplateColumn>
                </Columns>
            </MasterTableView>
        </telerik:RadGrid>

where Simple is the name of the used Skin.