I'm having the same problem. Here is what I did as a workaround. I copied the videos from my card to my computer and opened them using QuickTime. The from the file menu I exported them in at 1080p and then imported them into iMovie and it worked fine. Still at 1080p as I shot them. Silly to have to take this extra step. Maybe Apple will fix this in an update. I could swear I have been able to import my videos from the Nikon in the past.
Videos from my D3100 are imported into iPhoto along with photos. Those videos are then available (without any further conversion being necessary) in iMovie > Event Library > iPhoto Videos. I import via a card reader and not by plugging the camera in.
Videos from my Panasonic TZ7 on the other hand don't get imported into iPhoto but are instead available for import via iMovie > Import From > Dropdown. Strange.
In my case I would actually prefer all videos to be imported via iMovie as I prefer the way it creates Events for each import.