It's not exactly the same, no, but the motion signature from TRUE de-interlaced 60i should be the exact same motion signature as true 30p.
So it depends on how the footage gets de-interlaced. If you drop one field, and duplicate the other, then the motion should be completely identical (assuming identical shutter speed). But if the de-interlacing is some sort of blending fields together, then no, it won't be identical to 30p.
JVC intends to introduce a "motion smoothing" filter on their new HD100 for 24P and 30P footage; apparently it will shoot at 48fps and blend the frames together for 24p, or shoot at 60fps and blend the frames together for 30p, and they claim that this results in smoother motion. I believe they have patented the technique. It will be interesting to see how it looks.
60i is smoother because you're updating a field every 1/60 of a second. Although you're alternating odd and even fields, any motion on the screen gets updated every 1/60 of a second. With 30p, any motion only gets updated every 1/30 of a second.
