Hi Peter,
Yes and no. Yes you will be able to display SD video on an Apple Cinema HD 23″ Display via Multibridge Extreme, much as you can already do with the HDLink. However the main focus of HDLink has been HD and I expect the same will be true of Multibridge Extreme and Studio. SD video is interlaced whereas LCD monitors are progressive. We have an interlace simulation mode which helps display interlaced video on progressive LCD displays but sometimes it just makes sense to use an interlaced display, ie a conventional, inexpensive CRT monitor. Generally we find that many people like to display HD matieral on an LCD monitor but simultaneously output to their old CRT monitor for the best of both worlds.
You can display your SD at 2x scaling which makes it fill most of the 23″ LCD display.
Regards,
Luke Maslen
Blackmagic Design