If you're a *casual* shooter then I don't think you'll be happy trading in your HV-30 to get an "all-in-one" DSLR solution. Straight up.. we've been spoiled by the technology crammed in to the video cameras these days! By going the DSLR route you're giving up auto-focus, steadyshot, stereo audio, easy point-and-shoot convenience, etc.... Sure you have the option to shoot "photos" or "video", but trying to manually focus on kids running around, shoot action sports, or anything that moves will be a challenge in itself if you're not use to it. Not to mention if you want someone else that has never used a DSLR to film you wakeboarding.
I'm not up to speed on the video features of the Nikon D300, but I know from talking with Ronnie Romero that he has to do more to shoot video with his Nikon vs. the Canon 7D.. Even with the Canon you still lack the key features.. Auto-focus & steadyshot.
Just my $0.00002, but I think the whole video-DSLR movement is still too young for recreational users. Even with aspiring cinematographer it's a challenge and that's probably why you only see the "I just got my DSLR" videos posted online of people's pets, flowers in their backyards, etc.. and then nothing else.