1. For the last two posters before the OP: Negative Scanners are no longer commercially viable; especially for the professional marketplace. Hence there is no incentive for the manufacturers to update their software. Open source alternatives, if they do exist, just do not provide the functionality needed by a professional.
2. Partitioning your HD is indeed your best solution as long as this Mac remains functional. This solution is the so-called "dual boot" solution, which forces you to chose one environment or the other.
3. Once you are forced to purchase a Mountain Lion Mac (these Macs do not allow a SL partition), you can virtualize Snow Leopard (I use Parallels 8). After 18 months of instructing those in need of running Rosetta concurrently with Lion and now Mt. Lion
how to install Snow Leopard client into Parallels, Apple finally decided to help those out who need Rosetta, and re-release Snow Leopard Server for a 95% discount. This is especially true even though the Snow Leopard EULA does not prohibit the virtualization of Snow Leopard client in Lion or Mt. Lion on a Mac. However Snow Leopard Server is much easier to install into Parallels or VMWare Fusion.
Even though you do not need it now, my best advice is that you purchase Snow Leopard Server and put it up on the shelf for when that day comes for you. Apple is now selling Snow Leopard Server for $19.99 + sales tax & shipping costs at 1.800.MYAPPLE (1.800.692.7753) - Apple Part Number: MC588Z/A (telephone orders only). It may not stay on sale forever...
Here is a "testimonial" from a professional photographer who needed the ability to run his legacy professional hardware in Snow Leopard while upgrading (at that time) to Lion: