In both cases, the new firmware fixes an issue with manual exposure movie shooting. With a Canon 1D Mark IV running firmware versions up to v1.0.6, or a Canon Rebel T2i with firmware versions up to 1.0.7, the lens aperture could change unexpectedly with certain unspecified Canon lenses, including Macro lenses. This issue has now been resolved in the latest v1.0.8 firmware releases for both camera bodies.
In addition, the Canon EOS-1D Mark IV v1.0.8 firmware corrects some spelling errors in the French and Spanish-language menu system.
More details can be found on Canon's EOS Rebel T2i firmware download page, and EOS-1D Mark IV firmware download page, respectively.