Indeed Fed is playing great! His great run will ultimately end, when? I still don't know.. But to tell you the truth, I really though Nadal would beat Fed at IW and I got a really nice surprise when I saw the scoreboard... Miami? Well, I would think Fed has all the motivation to win Miami considering that if he does, that would mean he would be #2 again, so I believe that is a hell of a motivation. Can he accomplish it? who knows. Yeah, Roger is playing great, but you still got Djokovic, Nadal and even Murray to deal to win the title and that is a tough achievement that can be done but I don't think it will be easy..
Whether or not Fed can continue this run remains to be seen. I have seen his strategy of play and I really do like it, it gives him a lot of aggressiveness to his new style and what I did like the most was to see him go into the court against Nadal and stick to that plan for the entire match, that was new... BUT, I still think that if Rafa continues to hammer Fed's backhand every time he has a shot he has a lot of opportunities to beat Fed, so, yeah, I think that if anybody in the top 10 that is most likely to end Fed's run will be Nadal.
The thing is that Fed, even if he continues to play the way he is, has a lot of competition and any of the top 4 today could win vs Fed on any given sunday, as welll as the other way around.. I am just happy to see him play like this, I haven't seen Fed this "confident" since years back. This confidence might be what he was lacking and we will have to see if that is enough (apart from his talent, strategy) to get to the top again.
Will he ever be on top again? I, as a Fed fan, would like to say that I am confident of this, but I am not, it will be really hard work and if he could win SW19 he might have a shot at it... but its a long way to go still... we will see. I would be really happy to see Fed get to the top spot again (even if its for only 2-3 weeks) and break Sampras record, and win another GS while we are at it!

The good news for Nadal is that Fed is not on his bottom half of the draw, he is on Djokovic's part of the draw, the other good news, after watching the draws is that Murray and Djokovic have the hardest run to the finals on Miami, while Nadal and Federer have been given "cake walks" to the semis. Nadal should be happy about that, just as Fed. My prediction for the tournament for SF would be Del Potro vs. Federer and Murray vs. Nadal..
Anyway, we will have to wait and see how it pans out. Good luck Fed.