I recently changed computers and got a MacBook Pro which boots the kernel in 64 bit mode. The problem is, the only drivers Toshiba published for the G450 modem are 32 bit only (published on 2008).
So the only solution for now is booting in 32 bit kernel mode (pressing 3 and 2 when booting).
I seriously doubt that Toshiba are going to publish updated drivers; they were already rather unsupporting even when the modems were new. And Toshiba seems to have merged its mobile division with Fujitsu's... and even Windows 7 users seem to have problems. So... maybe this means that I should try to go back to the program-your-own-driver thing.