Same problem - HP 7400c software from HP web site that ran under Snow Leopard no longer runs under Lion, presumably because of lack of Rosetta.
Have also tried running the Windows XP version of the 7400c driver under VMWARE Fusion, and while the scanner and PrecisionPro software can be made to work, the scans are strangely lacking in sharpness, as if viewed through a ground-glass screen, and are therefore not useable.
Planning to try running a Snow Leopard virtual machine under VMWARE Fusion on Lion, to see if the HP's 7400c drivers for the MAC work better in this environment.
Like many other users I want to continue running a perfectly good HP 7400c - this scanner has a larger scanner window than most other more-modern scanners, and wiil therefore scan documents larger than A4.