In reply to Jon Wylie: I have always used the BMC insurance as they seem to have a good track record of getting people out of the sh*t and you have the peace of mind that they are mountaineering focussed.
I got a quote from Dogtag and whilst the prices are a bit cheaper than the BMC insurance it might be worth getting written confirmation that they would actually cover your expedition. Their documentation states that under the 'extreme' policy you are covered for "Mountaineering (organised climbs above 4,000 metres using ropes and all safety equipment)" whilst it specifically excludes "Free Mountaineering (without safety aids), Solo Mountaineering, Solo Rock Climbing and Freestyle Rock Climbing". I just wonder whether they would consider a non-commercial (unguided) expedition (or indeed an accident whilst unroped) to be excluded from their cover - the wording is certainly ambiguous.
Certainly worth looking into as the BMC High Altitude cover is quite a bit more than the Alpine/Ski policy if you're heading for a route other than W Buttress on Denali.