The low-end ones (they start at $380) have a single long bar of printheads that isn't replaceable, but the higher-end ones use replaceable printhead modules. All of them have both the normal printhead cleaning routines, and they have optical sensors to detect individual clogged nozzles. A clogged nozzle is dealt with in a variety of ways depending on where it's located and which type it is. The individual printheads are staggered and overlap a bit, and a clogged nozzle in the overlap area is just replaced by the overlapping one. A clogged black nozzle that isn't overlapped is replaced on a per-nozzle basis by colour nozzles, and if that isn't available, then it just relies on the fact at at 1200dpi, the dots spread enough to overlap anyhow and a single missing nozzle won't really show up on the page (you'd need two or more adjacent clogged nozzles to produce a visible streak).
It's important to note that these are all business and professional printers, though. They're designed for high volume where nozzles clogged from lack of use isn't a problem.