TCP sequence prediction was a hot topic around 2001, but most vendors have patched their OS quite quickly around that time. See this CERT advisory from September 2001, which includes statements from various vendors. Basically, if the TCP sequence prediction attack works against a system, then that system was not updated with security fixes from more than a decade, which almost certainly means that it has much bigger security issues.
To generate unpredictable sequence numbers, implementations are encouraged to follow the technique described in RFC 6528 (which supersedes the older RFC 1948). This is a custom PRNG with MD5. Cryptographically speaking, this PRNG kinda sucks: apart from using a hash function of questionable robustness, it also uses it "wrong" (that's a Merkle-Damgård construction; care should be employed when simply concatenating a key with some known input; HMAC should be used instead, as a building element); and the overall scheme is not very fast (a good stream cipher would be much better). However, speed is not a strong issue here (a low-end system will still be able to compute that algorithm millions of times per second; processing of incoming SYN requests will be the bottleneck, not the hashing) and we don't need really high cryptographic strength, since the attacker can "get lucky" with probability 2-32 anyway, so the PRNG needs not be better than that (typical goal for cryptographic algorithms is 2-80 or less).
Summary: TCP sequence prediction attack is a thing of the past; nice to know, but not applicable to systems which have been maintained for the last ten years or so.