DV Widescreen and HDV/HD have slightly different display aspect ratios (DAR), due to the way that pixel aspect ratio (PAR) is calculated and expressed in Premiere Pro. As a result, DV Widescreen's aspect ratio is slightly wider/shorter than HD's aspect ratio.
When you drop an HD clip into the DV Widescreen sequence with Scale To Frame Size set, it will decrease until it fits the entire image within the frame; this is why you see the black bars on the sides.
Though not quite as automatic as Scale to Frame Size--and especially if you plan to export to a DV sized output, like DVD--you can manually scale the HDV clips down to fully fill the DV Widescreen frame, essentially eliminating the black bars, by using the Motion > Scale parameter. It should be right around 45.5%; you'll crop a little off the top and bottom of the HDV, but it's pretty minimal. Save an effect preset for that Motion > Scale tweak, and you can easily apply it to other HDV clips; I've got a few of them created for using different sized assets in non-matching sequences.
Awesome! That worked out just fine.
Glad to help!
If you're really curious about what's going on under the hood (fair warning: it's all mathy and stuff), check out this blog post from Todd Kopriva with links to a number of articles: pixel aspect ratios in After Effects CS4 and other applications in Creative Suite 4 Production Premium « After Effects region of interest. Despite being on an After Effects blog, and from the CS4 version, the information is relevant to Premiere Pro CS5/CS5.5, as well.
Also, I'd be remiss if I didn't note that, should you ever be working in an HD/HDV sequence and wishing to export to something like a widescreen DVD, you'll encounter this issue again. Fortunately, there are cropping capabilities within the Export Settings window that let you account for this. Cropping a few pixels off the top and bottom of your image will allow the frame to be fully filled and eliminate the black bars.