Are you sure it’s the font that’s different and not the “Text smoothing” setting? The OS font is fixed (unless you use a third party utility to change it) and the font itself hasn’t changed for many versions of the OS. The thing that does change is how the text smoothing is handled and what the default setting is. You should have your SUTs set to use the same text smoothing setting (but they can’t all be set to “Automatic”). On 10.5 the options are Light, Medium, and Heavy. On 10.6 there’s only one option which is a checkbox for “Use LCD font smoothing when available”, but you can still use the smoothing settings from 10.5 via the Terminal as explained here.
Oh, and if you are using Text Images with eggPlant for Mac running against a Mac SUT, you should also make sure that the smoothing setting on the eggPlant machine is the same as the smoothing on the SUT. In that scenario, the Text Images are actually being generated on the eggPlant machine (not on the SUT) and will be using the font and smoothing settings of the eggPlant machine.