b43 is a kernel module, not an rpm package. You don't remove it with yum. There are only two Fedora 12 rpm packages that begin with the characters "b43" and will be removed by twohot's suggestion. One is a utiilty and the other is firmware. The b43 kernel module will survive that yum command.
remove the firmware that you installed for b43. Simply manually delete the folder /lib/firmware/b43. However, the firmware files rarely cause any trouble just sitting there if you decide to try another Broadcom driver or driver method. Every now and then, the b43 module has to be blacklisted if it is being automatically loaded when it is not wanted. But that can happen whether the firmware files are present or not.
True enough, the Broadcom 802.11 STA Linux driver is available from RPM Fusion in a precompiled yum-installable version (the kmod-wl and broadcom-wl to which twohot referred in post #4). But it officially supports only four Broadcom wireless chipsets. It "unofficially" is known to work with two additional chipsets. But there are many Broadcom wireless chipsets commonly discussed around here. You haven't even mentioned your chipset yet.
Of course continue doing what you want, but it is sort of standard procedure to identify the chipset being used by your wireless card and use that information to choose a Broadcom driver or driver method known to work with that chipset. It's more orderly anyway. Most of the Broadcom wireless chipsets commonly discussed around here are known to work with one or more currently available drivers or driver methods.
Now, I don't know what was causing the odd problem you described in post #3. Sorry. But if you want to identify the chipset of your wireless card, that Dell Mini-card almost surely will be using the PCI bus. Its chipset will be identified in the output of this...