I think that’s what a lot of people do, myself included. It's only unsafe if you don’t encrypt your wallet, else.. it's totally safe.

1. Even if somebody gets to your computer and accesses your Dropbox files (chances?), they won’t be able to do anything with the wallet.dat file anyway.
2. If somehow the wallet.dat gets deleted from Dropbox, you can always recover it on Dropbox's web interface.

I can't see why people would go through the hassle of creating Ubuntu-boot usb keys. It seems just as risky as having it in your Dropbox. If your key gets lost, or corrupted you’re going to have a bad time.

/Ledningen