lately I've noticed that the pressure I have to apply to the brake pedal has increased and quickly feels like I have hit a physical limit, but the brakes are just barely holding the car in idle. It's not all the time, and seems to be when I've been driving a bit. The pads are all in good shape and the fluid is ok. I've had many different break problems in the past with different cars, but I've never experienced this kind of failure before, and it's scary thinking you can't stop the car. Any ideas?

Did you get a recall letter? There is a topic on that here somewhere.

Does the brake pedal go down all the way?
If not, when you hold the brakes firmly and continue to hold, does the car slow down rapidly?

This definitely sounds like the symptoms of the brake vacuum line / brake booster recall issued for V-8 e60's, i.e., your '05 545i. It sounds like all brake vacuum assist is absent, the hard brake pedal issue. Before the recall, I would occasionally lose vacuum assist when the car was parked. Then, when I had to use the brake pedal during start (on an incline with hand brake not fully set), there was virtually no pedal travel, and it required a lot of pressure to get any braking action at all. Quite unnerving. Can't imagine the reaction if this would have happened driving at any speed. I'd say you need to get this to a dealer without delay.

any updates on this?
i think i might have the same issue.. my symptoms are that i lose brake power especially towards the end of a hard stop. the brake pedal would travel to to where it cannot be pressed anymore (not to the floor.. the travel is maybe like 3-4 inches). come to think of it, i can't get the abs to come on when driving which is weird because all of my other cars can do that w/ out an issue.. i've had the recall done.. they replaced the vacuum lines only.. i'm guessing my brake booster is suspect..

2 questions:
should i be able to get the abs to come on in dry while driving like 40mph?
does the pedal only travel like 3-4 inches then stop completely?

going to the stealer tomorrow to try to get them to cover it under the brake recall.. (out of warranty as of march)

ABS (Anti Lock Brakes) helps with braking by pulsating the braking (while you are holing the brake pedal pressed) to ensure the wheels are not locked while hard braking. Anti Lock brakes unit can be felt working when you brake hard on a wet/snowy pavement. If the road is not slippery, ABS won't kick-in unless you are driving really fast and brake hard.

thanks for the ABS definition..
better question would be, can you lock up your the wheels with the brakes when you're going 40 mph so that the abs comes on. if i remember correctly, it's not much of a problem on most cars.. i can't seem to be able to do that now..

i had to have my booster replaced for this. happened one week before the recall was announced. what has happened is the vacuum line check valve failed and let engine oil get in the booster.