I think just using a regular multiline Textbox should work:
<asp:TextBox runat="server" ID="txtMyTextBox" Rows="10" TextMode="MultiLine"></asp:TextBox>
This works very well yes. It actually took me a very long time until I managed to get it completely work, there doesn't seem to be too much info about ASP and TinyMCE for newbies like me. (atleast not in documentation and on this forum)
Anyway. For ASP 2.0:
Put these in your .aspx file
mode : "textareas",
theme : "advanced"
<asp:TextBox ID="MyTextBox" runat="server" Rows="10" TextMode="MultiLine" Width="557px" Height="267px"></asp:TextBox>
After that you can simple use the MyTextBox textbox in your class file (.cs or .vb.. whatever) Your output can be found in MyTextBox.Text (if you added a submit button or use the save function of TinyMCE + PostBack)
I know it's easy as hell, but it took me over 2 hours to get this working. So I decided to take 10 minutes to post this, which might help people out in the future.