No IT employer in Australia that I've communicated with has ever mentioned the
ACS in regards to qualifications, not once -- I would be surprised if any even
know it exists
It says a lot if you prefer to tell us you're an ACS member rather than that you write code for fun at home.
And this is only the first step to apply for an 'invitation to apply' for a visa.
What are my job prospects as a US software developer seeking to emigrate to Australia? How often do Australian companies sponsor visas for overseas workers?
Frankly, assuming you have the right to live in the US, you are much better off staying in the US if you want to maximize your opportunities as a software developer vs. moving to Australia.
I say this as a Canadian who worked as a software developer in Southern California for 4.5 years but moved to Australia two years ago, largely to escape the restrictions of US work visas and try my hand at doing my own startups.
Sydney and Melbourne are the only two cities in Australia with sizable markets for software developers. Compare that to multiple regions in the US with large software industries (SV, SoCal, Texas, Research Triangle, NYC).
There are very few pure technology companies in Australia so the vast majority of software development is in-house development for banks, insurance companies etc. Australian companies also tend to be relatively conservative in terms of technology platforms so PHP, .NET and Java are the norm vs. Python or Ruby.
Certainly there are very talented and capable software developers in Australia, but the software industry is so small and under-valued (compared to e.g. mining or real-estate development) that there's very little opportunity to advance professionally.
Check out http://www.seek.com.au/ to get a feel for what jobs are on offer.
http://www.realestate.com.au/ is also interesting to get a feel for the eye-popping prices that Australians pay for housing. (I rent.)
It's a shame that there isn't a well-defined exchange program between countries like the US and AU. In all other aspects I meet the visa criteria head-on, yet the fact that I don't have a degree will slow me down by 2-3 years.
As for your job prospects, they're excellent in Melbourne or Sydney, and good to fair elsewhere.