A number of you came up with some theories and then Chris said he had seen the ads at Liverpool Street with the name Baker Street in them. I've been at an event all day called 2gether08 (more on that soon) which was held in Shoreditch, so I came home via Liverpool Street & completely forgot about Chris's comment until I found myself in front of the advertisement.
Lo & behold there was "Baker Street" in the ad complete with roundel and everything. So what's that all about? There's a couple of replacement allergies too - chewing gum replaces leather (although how you'd be allergic to chewing up on a platform floor unless you were wandering around with bare feet, is a bit of a puzzler - if you were wandering around with bare feet, then allergies would be the least of your problems). Mould spores replace moisturiser.
The client or advertising agency must be testing this. Perhaps there is higher recall with a station name. Or maybe without it, as it looks weird? Any more theories on this?