Previous project was 1080 from a digital camera.
When I imported a previous CS4 PAL DV project into CS5, all the sequences were set to 1080 not DV.
I added a DV sequence, gave up and tried again. This time the sequences were DV as they should have been originally.
This suggests that there might be an anomaly sometimes when importing projects from CS4 to CS5 when they are of different types from the immediately previous work in CS5.
Working OK now, but posted so there is a record in case others have the same problem.
If I can repeat it, or others report the same issue, I'll certainly put in a bug report. But as of now it could easily be a one-off from a particular project.