A new variable clock speed Nexus bitstream for 1525 cards has now been released:
http://zetheron.com/Downloads/1525_Nexus_Variable_A.zip
This bitstream was put together fairly quickly to add in the variable clock speed and safe startup/shutdown features. It has 3 nexus cores and at 570MHz it can hit around 1.7GH/s with proper cooling (hash rate is 3 x clock speed). An improved 670MHz build (2.01GH/s) bitstream is currently being tested for release soon, and the 4-core nexus (2.4-2.5GH/s) bitstream is also coming fairly soon.