Nannette Thacker post an article on "Spam Catch without Captcha". http://weblogs.asp.net/nannettethacker/archive/2008/03/16/spam-catch-without-captcha.aspx
His idea is to use a hidden field that is irresistible to spam bots.
This is a similar trick that I also use to eliminate spam, where by i place a textbox with a label inside the form. If your doing this, add a little accessibility to the solution.
ASP.NET
<div style="position: absolute; left: -1000px">
<asp:Label ID="lbl_spam" runat="server" Text="Required, assuming that only an automated bot will see this. Elsewise, please ignore"></asp:Label>
<asp:TextBox ID="txt_spam" TextMode="MultiLine" runat="server"></asp:TextBox>
</div>
For everyone else:-
<div style="position: absolute; left: -1000px">
<span id=" lbl_spam">Required, assuming that only an automated bot will see this. Elsewise, please ignore
<textarea name=" txt_spam" rows="2" cols="20" id="ctl00_cph_main_txt_spam"></textarea></span>
</div>
So if your handling the response:
txt_spam has content = spam
Unlike Nannette approach, I prefer to mark the mail and forward it to a dedicated spam account, just in case.