When considering the 20% threshhold, exactly how is it being counted? For example, A plan has 9,000 total participants, but 7,000 are already in pay status or termed vesteds. If 600 of the remaining 2,000 active participants are terminated due to downsizing, is that a partial termination? It's 30% of the actives, but only 6% of the total particpant base. Is there a clear line or is this something that is still the discretion of the courts?

http://benefitslink.com/IRS/revrul2007-43.pdf
This Revenue Ruling concerns a DC plan. However, the text uses language that suggests that a percentage test is viewed based on active participants, such as "severance from employment" or "turnover rate". IMHO, this is the common sense approach.
As always, and as mentioned in the RR, any partial termination is evaluated in light of the particular facts and circumstances.

Thank you!