I have a 25 minute documentary project edited on Premiere Pro CS5 which I plan to write as a Blu-Ray disc. The original footage is a combination of HDV footage from a Canon XL-H1A camera, and AVCHD from a Canon 7D. I'm not sure whether it's best to set up my Encore video transcoding settings as 1440 x 1080 or 1920 x 1080 before I import the footage into Encore. I'd welcome advice on this.

Neil

Is the mixed footage combined in the same timeline, or is each camera imported as a separate asset?

If they're separate assets, on separate timelines, then let Encore encode them each in their native formats (ie automatically)..
If you want full control over bit rate allocation, then create your own presets, and allow for the native resolutions to be encoded as mentioned above.

Good luck!!!