At this point, I don't think the combination of RIM/Nokia/Microsoft would substantially lessen competition in the cell phone industry. No regulator stops an acquisition if one of the companies is going to die without it. It's easy to show that with RIM, a Nokia acquisition would be harder to get past European regulators.
Still, the combined market share of all three for smartphones is ~15%. That isn't the kind of number where regulators step in.With dumb phones, RIM and Microsoft don't really compete for that market, so Nokia's marketshare there wouldn't be a problem. So on a regulator front, they probably would be able to go through with the merger. If Motorola went through, i don't think they are going to stop anyone but Apple from acquiring phone companies.