While you may not be able to get them refilled yourself, HP does run a recycling program for them, free of charge: HP Product Return and Recycling.
Didn't look in to whether they simply refill and resell them or if they actually melt the cartridges down and start from scratch... either way it's better than the landfill. Unfortunately, you're still stuck buying new cartridges.
runs ran a similar program that rewards you with a free cartridge for every 5 you send in to be recycled... better than getting nothing in return. They also give you a free one for every 5 you buy. UPDATE: Lexmark has ended their rewards program:
Thank you for your interest in the Toner Rewards program. At this time, Lexmark is no longer accepting new enrollments into the program.
As far as companies go, HP appears to be near setting the bar for environmentally friendly electronics: Greenpeace Guide to Greener Electronics 2017.