The good news is, as Danny Princz noted, that Pixel Bender Toolkit V2 from Adobe has the replacement for 3rd party unmult filters for CS4+. It will install "Alpha from Max Color" 32-bpc effect under Utility effects menu. It should work in Photoshop too.
But you don't really need a plug-in.
Chris Meyer (steps outlined in bonus tip), Brian Maffitt, and Dennis Wilkins also reminded us of other solutions, for instance the Set Channels effect, with the Use for Matte popup set to Maximum or Luminance, followed by the Remove Color Matting filter, all of which can be saved as an Animation Preset for quick application.
You can find nearly the same recipe from Todd Kopriva (leaning on Trish Meyer) in AE Help: "A way to get the same result as the Knoll Unmult plug-in using the effects built into After Effects is to use the Channel Combiner effect, with Max RGB as the From value and Alpha Only as the To value, and then apply the Remove Color Matting effect."
See also Motion Graphics Exchange Removing black or white from layer/footage, deMult/unMult.
Update: Jeff Almasol from the AE team added "FYI, the AE CS5 installation also includes the Pixel Bender Toolkit 2 component. To get Alpha from Max Color and other samples to appear in AE CS5, launch Pixel Bender Toolkit 2 once, then relaunch AE." (You can double-click a .pbk file to launch)
Also, Red Giant followed up quickly with Knoll Unmult: free AE filter.