Identicons for bitcoin addresses have been discussed before:

https://bitcointalk.org/index.php?topic=122240.0

In short it works fairly well as an extra checksum but it has much less entropy than the full address so matching the first few address characters and the icon is not that hard.  It depends on your use case how useful it really is, it can help with easily identifying mistyping of addresses (but so can the built-in checksum), but doesn't necessarily prevent spoofing one address with another similar one.