Have to agree with all of the disagreements to this. The 404 is a 404 for a reason, just like 301 and 302 are different for a reason. It's not uncommon though for WordPress to do things like this, or blogs for that matter. If an author changes the title or date of their post, and the URL structure is reliant on those two pieces of data, then the URL will change. The old URL is preserved in a DB and if accessed again, 301s to the newly named resource. Others, will throw a 404 and give a cutesey Levenshtein message, "did you mean x?" at which the user can decide to go to the new resource. It's all circumstantial... It shouldn't be enforced.

Re: Google and PageRank, pretty certain they've addressed this and recognize 302 and 301s and treat them the same. Previously, this was an issue.