What about PAL video converted to NTSC? Any problems there?

Depends. It's much easier to go from 25p to 60i through pulldown without affecting speed, but then you're losing resolution (576 down to 480 lines). You could slow down from 25p to 24p, but then you have the inverse problem of 24>25 conversion - you slow the film down by 4%, lowering the pitch in the process, too.

This does present a slight issue for Blu-Ray, even in PAL countries. First, Blu-Ray doesn't allow for 25p, so all PAL-speed BRs are 50i. For an NTSC country release, you either have to slow down to 24p, or do the same 25p>60i pulldown conversion I was talking about earlier. Which is fine, but again, it's interlaced.

So for 25fps stuff on Blu-Ray, it's going to be 1080i unless you slow it down to 24fps. This is why stuff like Planet Earth and Life is 1080i on all releases, and why some recent US Doctor Who releases (like Sperhead from Space) on Blu-Ray are slowed down to 24p (the UK release is 50i).

Really, I don't understand why 25p isn't in the Blu-Ray spec, even in NTSC territories. It's digital video, it's not hard to show 25p on any modern HDTV (50Hz or 60Hz). Maybe with the 4K spec revision that's supposedly coming, they'll add support for 25p (and 48p for that matter - I really want The Hobbit in 3D 48p at home, that was an awesome experience in the theater).