Okay, I spoke to dr_frost_dk and it sounds very likely that my current N900 (that I bought from him) never had R&D Mode turned on ever.

reinob: Did you get around to testing it? Any problems or need for more detailed instructions?

@ Everyone else: Tests on FRESHLY-reflashed N900s that HAVE been in R&D Mode before the reflash but haven't been put into R&D mode since, are also appreciated.