Same problem with mine,
This might help, I'm going to follow these instructions at the weekend.
This process will take an hour or so, but it will fix the leak.
The problem is the drain in the cowl in front of the windshield (which is also the air intake for the AC system) is clogged. When the drain clogs it runs in through the fresh air vent, through the blower motor, and onto the floor.
You need to remove the plastic parts that cover the air intake. It will help to raise the hood to the verticle position. There are screws, plastic clips, and as I recall, little plastic buttons that hold those pieces in. You need to remove the whole thing, as the drains on the driver's side are probably clogged too.
This is the piece that you are removing:
http://tad.grosvenor.org/gallery/C280Mods/IMG_3051
The most important drain is right behind the right front wheel, in the engine compartment. This tends to collect leaf litter that sifts its way through the plastic cover on the intake. I used my finger, the hose, and a bent coat hanger (to get to the vents under the wiper motor) to clean the drains on mine. Once you get the cover off, you'll see the air intake, and the drain.
This is the bottom of the air intake box. You can see the pine needles in the drain on this one. Be sure to clean the debris from the area outside of the drain too. On mine it all flushed out onto the ground without too much effort.
http://tad.grosvenor.org/gallery/C280Mods/IMG_3053
-Tad
Thanks to each and every one who replied. I followed Jimmy's and Sym's instructions, and, as many of you had suspected, the rubber plug at the bottom of the air box was blocked. When i partly lifted the cover of airbox i could already see about 1 and half inchs of water (at its deepest). I used a bendy stick to clear the rubber plug. Poured water on the windscreen to test, and true enough water did not drop into the footwell, but went through the rubber plug. Note there was just a plug at the bottom of the box, no hose or anything else, so that water was simply being drained into the engine bay then the dripping into the ground/street.
I opened the panel under the glovebox and the foam inside was wet (from before) but little else. left it open to dry. I plan to re-do the above procedure later this summer and enusre its thoroghly clean. Is there anther plug at the other end of the box or is the passenger side one the only one ?
Now that the cause is known, would any damage have ocurred to the system(eg air con/heating system, pollen filter, fan etc ?) due to water leaking in ?Would it be worthwile not to use the air con system for few days waiting for things to "dry out" ? or is this pointless ?