As of July, it looks like a popular blacklist used in default SpamAssassin installations is no more. Users were reporting false positive issues, where every message checked by SpamAssassin would receive a score of 2.43, supposedly due to the sender being listed in the blacklist bl.open-whois.org.
The Open Whois Blacklist appears to have been created in 2007, with a goal of promoting transparency in domain registrations. According to the (now deceased) website, "It is a list of domains which are privately (or anonymously) registered, e.g. through services such as Domains By Proxy, or Moniker Privacy Protection."
As of July 18, 2009, it appears that a squatter has taken over the open-whois.org domain name. At first, the new owner of the domain used a "wildcard" DNS record, resulting in the return of a positive response for any DNS query. The net effect is that every domain checked against this blacklist results in a DNS response that makes your spam filter think that the domain is blacklisted, usually incorrectly so.
Since the issue was first observed, the squatter must have noticed all of this DNS traffic coming from SpamAssassin users and decided that the traffic was undesirable, so they've modified the domain in whois so that its name servers point at obviously invalid IP addresses.
That's good, because it means there shouldn't be any more false positive issues, for now. But, it does mean that your SpamAssassin checks take longer than usual, as queries against this dead blacklist will time out. (And who is to say the squatter won't resurrect the domain with valid DNS servers and perhaps another DNS wildcard, causing a whole new batch of false positives for a whole bunch of SpamAssassin users.)
If you're a SpamAssassin user, it would be wise to remove or disable the SpamAssassin rule that check for that blacklist. The rule you're looking for is located in the "72_active.cf" file in the rules subdirectory of your SA installation.
To disable this check in your SpamAssassin installation (manually), move or delete the "72_active.cf" file from your rules directory. Where this directory is exactly located is going to depend on your installation. On my friend's Linux installation, the directory path is /etc/mail/spamassassin/rules .
The better thing to do, I was advised by friendly SpamAssassin user Phil Randal, is to run sa-update. It's best practice for SA users to run sa-update every week or few to load the latest "in between-release" updates. Running sa-update will ensure that the bl.open-whois.org check is disabled.
I suspect that this blacklist check will be removed from SpamAssassin in future releases, but as of today (8/18/2009), the check is still in the most recent version available for download (3.2.5). As long as you run sa-update or manually disable this check, you should be all set.