Final Cut Pro 5 supports capturing a number of different media types using FireWire device control. Because of differences in the data-structures of DV and HDV, there are some important points to keep in mind if you plan to capture DV video that is being down-converted on-the-fly from an HDV tape source.
Some HDV decks and camcorders are capable of playing an HDV tape and performing a down-convert of that signal, sending a DV signal out via FireWire. Final Cut Pro can capture this DV signal, but regular DV device control may not be appropriate in this capture scenario, because the tape transport mechanism is not really handling DV-formatted video on tape.
DV-formatted video features intra-frame compression; that is, each frame contains all the data needed to represent that frame. HDV uses more complex compression methods, and it's necessary for several frames of HDV to be read into memory before they can be displayed or captured. A single frame of HDV video may contain information about the picture content of neighboring frames, while not containing an entire "picture" of data itself. (HDV features an MPEG-2, Long-GOP architecture.)
Because of these differences, HDV and DV use different device control methods for communicating with the deck or camera during capture. If there are timecode breaks on the tape and Final Cut Pro's preferences are set to "Make New Clip" on timecode breaks, unexpected results may occur.
To experiment with capturing DV from an HDV source that is being down-converted on-the-fly, it's a good idea to change the setting in User Preferences for "On Timecode Break" to "Abort Capture" or "Warn After Capture", as opposed to the default, "Make New Clip".
If you use Capture Now and the preference is set to "Abort Capture" on timecode breaks, you'll have to attend to the process since the capture will stop after each timecode break. The upside of this method is that you will maintain original timecode, allowing you to re-capture later if desired.
If you use Capture Now and the preference is set to "Warn After Capture" on timecode breaks, you can capture the entire tape at once, but the timecode of the captured result may not match what is on tape. This will preclude the possibility of subsequent recapturing based on timecode.
Another method would be to capture the source as HDV, and then use Compressor to perform the down-conversion to DV.