1) Is it possible to create a working hours report for a week?
Excellent question. There isn't any easy way to do this today, but this is absolutely a high priority enhancement for the next release targeted for 2H08. There is a workaround - see suggestion offered in #2 below.
2) The only way I can think to do it would be to use scheduled maintenance to limit the reports to working hours (not preferred) will the results be averaged over 24 hours or just the enabled polling hours?
That's definitely one way of doing it, but that means you are turning off alerting during that timeframe. Alternatively, see suggestion below from tbaker:
What I did was modify the time selections in the report. My busness hours are from 8AM-2AM so I modified the time navigation rules to go back 16 hours and I changed the time selection to include 18 hours. The report runs automatically at 2:15am and gives me the availability percent of that time frame.
3) Or can you export a weeks report as txt or csv?
There is no ability to export to CSV, but this is something we are also evaluating for future release.