In reply to Geoff F:
A road bike would definitely be faster for the commute. It depends what you're used to. I commute 6 miles from the centre of Edinburgh on my "relaxed" road bike, but I'm used to a racing geometry. If you've only ridden mountain bikes it would probably feel really stretched out.
I use a backpack but I don't carry much stuff, just lunch and a folder with some paper. If you're carrying a change of clothes panniers start to become more sensible. If you're looking at road bikes be aware that many modern bikes do not have the mounting points for panniers.
Whatever you end up with, a rear mud guard is critical. Make sure you get one that fits inside the rear triangle rather than one clipped to the seatpost. Front ones are less effective (your feet will get wet either way) and I managed to rip mine off with my foot. My toes barely cleared the tyre without the mudguard, with it there was no hope!
If you do go with a road bike I'd suggest an aluminium frame, deffo second hand. I have a Trek Alpha 2.1 which is probably slightly overkill but it is very comfy and fast! People buy reasonable bikes in Edinburgh and then realise the roads are bumpy and it is "weathery" a lot and give up riding so you can usually find a good deal.
The more alu (less steel) you have the less rust you'll deal with. Also many recommend stripping the paint and making it look crap so people will be less likely to steal it. Depends where you lock it up.
With a road bike your maintenance will probably be higher because the parts aren't designed to have as much forgiveness as a mountain bike or hybrid with regards to muck. I usually end up doing a thorough cleaning once or twice each winter, but I fiddle with bikes as a hobby so its fun.