Apple doesn’t tell you this, so we will.

With OS X Lion, the new operating system for Macs released Wednesday, Apple (AAPL) has just cut a significant tie to its past — namely legacy programs like Intuit’s (INTU) Quicken that were written for the old PowerPC.

As John Siracusa points out in his definitive and thoroughly entertaining Ars Technica review:

“Though this is the second version of Mac OS X that doesn’t support PowerPC processors, this is the first version that won’t run PowerPC applications. In Snow Leopard, the Rosetta translation engine allowed PowerPC applications to run, and run well, often faster than they ran on the (admittedly older) PowerPC Macs for which they were developed. Lion no longer includes Rosetta, even as an optional install. (link)

What this means is that once you upgrade to Lion, any applications that haven’t been re-written for the Intel (INTC) architecture will stop working.

For many programs, this is an inconvenience easily rectified by buying the new version. For Quicken, it’s a potential disaster.

Nowhere in Apple’s press release or on its (currently nonexistent) OS X Lion support page does it warn Quicken users or tell them how to prepare for the upgrade. [UPDATE: The Lion support site is now live, but its section on incompatible software does not mention PowerPC or Rosetta. Nor does it warn users that they might lose irreplaceable data when they install Lion.]

If you have financial data you want to preserve, we strongly suggest that before switching to Lion you back up your hard drive and transfer your Quicken files to one of the many new financial planning programs that can read them. See Exit Quicken, pursued by a Lion for some suggestions. Some are free. None are perfect.