"USB device drawing too much power" error on startup with no devices connected
It could be a corrupt USB kernel extension.  I'd try booting to Safe Mode:


Turn off your Mac.
Turn it back on and before you see the Apple logo on your screen, press and hold the Shift key until the Apple logo appears.
A progress bar should appear below the Apple logo - during this time your Mac is performing a directory check and loads only necessary Apple kernel extensions (among other small things).


A feature of Safe Mode is that it reloads the kernel extensions from scratch, instead of from a cache file.  It could be that during the Lion upgrade a kernel extension was damaged and has been cached afterwards.

Try the Safe Mode boot and see how it goes.

Good Luck!