In fact, manufacturers are supposed to be moving to the new NMRA standard for the PluX interface
. The 21 pin interface was only an interim standard and is now due to be phased out by 2010. Of course, the 21 pin connector is quite popular, so it is likely to be around for a while.
The new PluX connector will come in 8, 16 and 22 pin varieties. The good thing about this new standard is that it also defines the physical size of the decoder, so you will know that the decoder will fit in the space available, if the manufacturers follow the standard (I hope Hornby are reading this!). So long as reasonable room is left for a speaker to be fitted, this should make decoder fitting a breeze in the near future.