How can I install/run apps that use Rosetta on Lion? Is there a workaround to install/use apps that are using Rosetta on Lion?

I'm still using Zend 5.5 and I don't want to switch to a new IDE right now.
At the time of writing Rosetta is not included with Lion, not supported by Lion officially and not available from any other channel (official or unofficial) for use on Lion.

So: no, there is no workaround. Your Rosetta-based applications cease to function if you upgrade to Lion.