Redeeming a private key is the same as redeeming a Casascius Coin.
Simple method:
1. Create a throwaway wallet at Blockchain.info
2. Import private key(s)
3. Send your funds
Takes less than 5 minutes in most cases.
Thanks for the pointer Mike. I'd not really payed much attention to blockchain.info's wallet. Seems pretty good. I like instawallet slightly better for certain things due to password convenience, but everything has it's strengths and weaknesses, and blockchain.info is certainly much more feature rich.
The ability to import wallets is nice, but it's to bad that drag-n-drop is the only method supported. I'd have to dick around with my preferred windows manager on my primary workstation to support it. Most drag-n-drop UI's also have a select file option, but not blockchain.info that I can see. Funny because I would have anticipated a general correlation between those with interest in Bitcoin and those running fringe (and often more secure) operating systems.
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In other news, firing up my ancient client worked OK for receiving some BTC from my instawallet, but did not seem to send very appropriately (judging from blockexplorer.) Probably associated with transaction fee settings, or who knows. I'll wait a few days to see if the missing value materializes.
Edit: Update: I was reading blockexplorer wrong and my send transaction off of the old client did work as expected.