There is really no excuse for this slice of gratuitous cuteness. I use it as the background picture on my iPod Touch.
Having these two around actually had me wonder why some cats clean each other. After all, cats have the capacity to clean themselves, and usually they don't need help (from people or from other cats).
One person I asked mentioned that it was a behaviour signalling the "dominance" of one cat over the other (i.e. the one doing the cleaning was expressing dominance over the one being cleaned). Some cursory Internet research tells me that in fact mutual grooming is usually a form of bonding between cats, where there is already close relationship; one site stated that "many times, if there is an age difference between the two cats, the older one will "mother" the younger one (even if the older is a male), and groom it as a protection tool. If they are the same age, it's more directly related to bonding". So grooming is seen as a form of "cat communication" that tells us the cats are getting along (no surprises there).
These two big babies were born in the same litter. Bingley's been cleaning Darcy since they were quite small, as the video below demonstrates--they were under three months old when it was recorded.