Have you considered just renting a lens? It would satisfy your equipment needs for a once in a lifetime event, but still not break the bank. There are a number of companies out there, but the only one I can recommend is:http://www.lensrentals.com/rent/canon/l ... rtelephoto
They have a fantastic inventory, great prices, great service and wonderful folks to help you. (They're also less than 5 miles from my house!)
One other question, are you sure you need that much length. The only person that I know of that shoots bears in Alaska on a regular basis is one of the members on Pbase. For most of her bear shots in Alaska, she uses a 300mm f/4 IS lens.
Here's a link to her site: http://www.pbase.com/britestar/wild_gri ... s&page=all
Hope this helps.