Is PHP the only choice re: massive and rapid uptake of a re-deployable, extensible web application?
As far as points #1 and #2 go, you are probably right. No other platform is so widely, easily and cheaply available in terms of hosting companies and packages like the LAMP stack. Plus, most incompatibilities that can occur when deploying an application to a completely unknown web space are well documented, their number is limited, and can mostly be checked beforehand (register_globals, safe mode, allocated script memory, etc.) 
If I were thinking about developing a web application that I want to see spreading as quickly and as far as possible also among non-professionals and end users, PHP would be my platform of choice for these reasons. I must add that I am deeply familiar only with the hosting market in Germany, but I'm quite sure the basic characteristics are the same.
As for developer availability: People who claim to be able to speak PHP are easy to find. Those who will actually do a good job for you, less so. Still, I think it is safe to say that PHP developers are easier to find than, say, Pythonists or Ruby developers.
I don't expect this to stay this way forever, though. Other languages are gaining popularity, and in the end, developers and which languages they like influences the hosting market massively in the long term.
My own answer to this question is YES, but I'd like to hear from others.  Put another way the question could be: Would the success of 1-click-install WordPress (not WordPress.com, which is SaaS) be possible if it weren't written in PHP, all other things being equal?
The critical associated requirements I believe support PHP are:

hosting/deployment flexibility
developer reach
flexibility and depth of knowledge around server performance tuning

Items #1 and #2 are equally critical, and both are far more important than #3.
FWIW, I'm not a particular fan of PHP - can anyone truly be? - but the goals of re-deployment and extensibility point wherever they point.  Please, please do not pollute this space if you do not grok the question.  This is not about PHP and it's relative merits, or lack of merit, compared to other programming languages out of context.  I am looking for insight specifically around language choice as it relates to deployment/uptake/extensibility strategy as outlined.
Thanks.