In a few circuits, preliminary injunctions are decided using a sliding scale test: the higher the likelihood of success, the less irreparable harm need be proved, so when success looks like a sure thing, sometimes only the "possibility" of irreparable harm will suffice.
Not any more. In Winter v. NRDC, No. 07-1239 (U.S. Nov. 12, 2008), the Supreme Court rejected the notion that preliminary injunctions may be entered based on a showing of only the "possibility" of irreparable harm. (Slip op. at 12.) Instead, the movant must make a "clear showing" on each of the four relevant factors.
Historically, the circuits' varying preliminary injunction tests often play a role in deciding where a trademark plaintiff should file suit. This decision, however, should -- if the circuits actually cite and follow it -- go a long way to eliminating this as a forum-shopping concern.