A US court (sorry not knowledgeable about EU antitrust) may impose (F)RAND licensing if the holder of the patent typically licenses the patents under a (F)RAND scheme but blows the ND (non-discriminatory) part .
For example, if Phillips typically licenses all its DVD patents under (F)RAND terms to DVD reader manufacturers, but then suddenly decides to charge Apple 100% more to make their own DVD reader drives, a court may force Phillips to license those patents to Apple under terms that are less discriminatory.
This is a gross simplification, but hopefully it's helpful. US courts don't force FRAND terms out of the blue. A patent, at it's heart, is a right to exclude and forcing a company to license that right (when they have made no commitment to do so) isn't part of the patent bargain.
This (quick scan by me) looks like a great overview of current (F)RAND policy and pretty up to date with recent litigation/regulation.
DOJ just released a position paper on these licensing schemes yesterday. Interesting reading: http://www.justice.gov/atr/public/guidelines/290994.pdf
[Another update for posterity...]
On April 25, 2013, Judge James Robart in the Western District of Washington issued an 207-page decision that marks the first time a federal judge has tried to define what constitutes a fair, reasonable, and nondiscriminatory (FRAND) royalty rate for standard-essential patents. The decision provides a rigorous factual and legal analysis for determining a FRAND rate by modifying the factors traditionally used to determine reasonable royalty calculations in patent infringement suits and adapting that framework for the circumstances presented by patents that are essential to industry standards.