I would imagine that the ideal ratio of testers to developers would depend on the development strategy used.
In an XP strategy that emphasizes test-driven development, developers write tests first and all developers are in effect testers as well, at least at the unit level. In this process, relatively few pure testers are needed to test integration of components and systems.
By contrast, a traditional software development process does the requirements analysis, architetecture development and coding at the beginning and testing doesn't ramp up until significant portions of the system are coded. This process needs more testers, who both look at low and high level behavior. Because testing comes in at later phase, the proportion of testers needed may also change over time.