You are asking about an extension of a non-relativistic phenomenon into relativistic regimes. To do so, you need to first more precisely define all your terms.
In this case: how do you define temperature relativistically?
There have been many academic arguments about this alone. Relativistic thermodynamics can be a tough subject. I would start reading there, before continuing.
That being said,
Look at Planck's law (I'm too lazy to tex it up
http://en.wikipedia.org/wiki/Planck's_law )
Unless frequency transforms the same as Temperature, then the distribution will have a different dependence (and thus peak position).
Also, if something is moving relativistically, the emissions will have noticeable doppler shifts. Are the frequencies measured corrected for this? If not, then no Wien's law should not hold. Or do you mean to average over all emissions in all directions? (in which case I suspect the answer will still be no, but I haven't calculated it)
Basically, the question needs to be defined better, and while tracking down better definitions you will probably answer your own question. Please share what you find along the way in case others are looking up this question later.