From near the bottom:
"Some further studies failed to reproduce the original experiment's results, but in at least one of these studies[12] both caged and "park" rats showed a decreased preference for morphine, suggesting a genetic difference"
Still, it's an intriguing result. The reviewer comments from science and nature would make interesting reading - too bad they are not public.
But also, from the immediately preceding paragraph: "Several later studies did appear to confirm its findings — for example, Bozarth, Murray and Wise in 1989, also published in Pharmacology, Biochemistry and Behavior — but nothing came of those either."
That paints a confusing picture, so without looking at the specifics of the later studies for possible differences, I think it's hard to come to a conclusion.
Wikipedia isn't so good when it comes to drug related topics. Or rather, it is atrocious since the linked study (http://wings.buffalo.edu/aru/HOUSING.html) arrives at the completely opposite result Wikipedia claims. It's an interesting read because it also discusses why setting up a study to fairly measure voluntary drug intake among groups of mice is very hard.
After reading portions of that study, I find it hard to make meaningful comparisons across them. The cited source's two conditions seem to be fairly close in many respects IMHO:
"The first group was housed in individual stainless steel cages (18 x 25 x 18 cm) that prevented tactile and visual contact among the rats."
"The second condition consisted of rats housed in groups of 10 in a large stainless steel cage (45 x 101 x 39 cm) that permitted social contact; these rats displayed normal play behavior, dominance struggles, and social grooming."
Okay, one rat in 450 cm^2 vs 10 rats in 4545 cm^2, both in steel cages. Compared to Rat Park, which was 8.8m^2 (close to twice the size), and was specifically not a steel cage[1]. Then again, I doubt this study was done specifically to confirm or refute the earlier experiment, but to test further hypotheses in the same area.
Rats are social creatures so you can't just measure the area of their habitat divided by the number of inhabitants. The experiment setup would be analogous to humans living either in prisons or in solitary confinement.
In my previous comment, I cited another study (http://www.ncbi.nlm.nih.gov/pubmed/9148292?dopt=Abstract) that also failed to replicate RP. That time the flaw of that replication was possible inadvertently introduced strain differences among the mice. It's always possible to find some variable that is different and therefore claim that the result isn't relevant. I'd imagine that's one big reason why replication efforts is so uncommon in the scientific community.
Btw, BK Alexander has had over 30 years on him to setup a new Rat Park, have it monitored by other scientists, and prove once and for all that his original results weren't just "a fluke."
Still, it's an intriguing result. The reviewer comments from science and nature would make interesting reading - too bad they are not public.