I just found out about this one.
This is especially important for people running Macs with Tiger, who can't use Bryce 5, but anyone else who wants to upgrade, now is your chance.
All you have to do is join the ArtZone Community. You'll get a voucher to get the upgrade to Bryce 5.5 for $100 US off. (Since Bryce 5 is free, the upgrade is all that anyone needs.)
When you go to the page, and put it in your cart, it'll look like you're choosing an item that costs $119.95. But after you click the button to start the Checkout process, the voucher will automatically be used, and the price will go down to $19.95. So don't panic, and go ahead as if you were going to spend all that money.
Bryce 5.5 works perfectly with Tiger.
Hope this helps!