Hmm, at school we were taught that the maximum temperature is such that wavelength of the emitted black-body radiation would equal Planck length, is this rationale no longer sound?
Black-body radiation describes the distribution of power emitted at given wavelengths or frequencies emitted by an ideal black body.
1. Distributions don't have one wavelength. There is a "peak" wavelength at which most power-per-wavelength is emitted, but there is still power emitted at all wavelengths no matter what temperature something is.
2. There is not sufficient reason to believe that the Planck length is a limit on the wavelength of light. This would break Lorentz symmetry.
3. Most things aren't ideal black bodies, but still have temperatures. Even if very hot objects couldn't emit light of Planck length, they can still couple to the environment and emit heat in other ways.
... Also known as the Planck Temperature, which is the highest listed possibility for absolute hot in TFA.