It looks like Cisco have finally announced the SSC5 (as well as IPS v7) - an IPS module for the diminutive ASA 5505.
Not sure on pricing or availability yet, but it looks like it is missing some features from the full blown IPS sensors.
Basically, it seems to be missing anomoly detection, Virtual Sensors, Custom signatures, Global correlation (new in 7.0) and the ability to re-enable retired signatures. It is capable of 75Mbps of throughput, approximately 50% of the 5505’s capacity. It has no dedicated management port unlike the AIP-SSMs.

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May 15th, 2009 at 9:30 pm
at 200,000 Yen I’ll give that one a miss!
http://cisco.hako-net.co.jp/products/cisco_asa_ssc_aip_5_k9_.1.html