Actually I saw a neat program on last night on Thisoldhouse I think, anyway PBS about a new adhesive solution for large areas of plaster which had cracked and separated from the wood lath. It involved drilling through the plaster at various points along the lath, injecting the special adhesive, using pressure umbrella head temporary nails to pull the plaster sections in.... a whole system. You might check their website for details. I had previously just torn off whole areas and tried to match the sections, but that was much harder.
Of course where whole sections have already fallen out, the sheetrock option sounds more attractive.