Hi,
on the support portal (right hand side "Cisco IronPort Product Discontinuance"):
http://www.cisco.com/web/ironport/index.html
Here's the direct link to ESA hardware:
http://www.cisco.com/web/ironport/c_x_series_life_cycle.html
Dates for the C150 are:
End of Sales: January 01, 2010
Latest Renewal: December 31, 2011
End of Life: December 31, 2012
Cheers,
Sebastian