i might be wrong on some points but:
the original GoPro (non HD) was designed for 2gig cards max.
the firmware update basically installs a program that will allow your GoPro to accept larger cards up to 4gig.
other small things that the firmware updates; you can film upside down and still produce normal footage instead of upside down footage
i also believe they adjusted some smaller stuff such as metering, whitebalance, exposure settings etc...
basically finetuning the cam...
For the GoPro HD we're still waiting for the first firmware update; important stuff this update might solve; the memory card problem, the on-off problem, maybe even more finetuning of the whitebalance, exposure metering, but also allowing to record while plugged into your laptop or usb adaptor (interesting for shooting long timelapses...)
allowing for upside down filming,
etc....