There is no answer that is "right".
As much as I would love to say that the main influencing factor for most companies is the target quality of the application, however that is not my experience.
The single biggest factor I have seen over the years is that defines the ratio is available budget that the company is willing to spend, and a lot of companies value lower costs over quality.
I can give you some real numbers from my past projects for reference.
For me the highest I have ever seen in Australia is probably 1:3 (so one test per three developers), with 1:5 or higher (less testers) more commonplace. The types of applicaions being developed are large, corporate level commonly internal web based applications.
I think when you get to commercial software, then the ratios need to go up, as the number of tesitng configurations increases.
For example, If I am testing an internal only applicaiton I will typically only need to test on one browser. For a public, internet deployed site, you need to test many, many more.
If you are testing software on the iPhone and you need to target all devices ever shipped including iPod touches there is now something like 12 hardware devices, with three 4 major OS versions.
With that in mind, I tend to focus on delivering as much "bang for buck" from the available headcount that I have.
One of the biggest ways I have seen to tilt the scales in your favour is an effective test automation suite. The reason that is a big factor is that it can essentially "free up" your testers from manual regression to be more effective elsewhere.
If you are trying to justify a larger staff, I would recommend to use metrics to support your case that you need a higher ratio, then capture the results for future reference and justificaiton to your peers.