Flash frontend with a SilverStripe backend works very well. I used it for several sites already (examples: http://bold-werbung.ch, http://chemicalbox.com).
I never felt the need to build the login form in flash, but it is certainly possible. The easiest way is probably to examine the html login form and use the same form-action and field values in flash.
As far as I know, development on AMFPHP has stopped. They have a good AMF implementation over at zend though (http://framework.zend.com/manual/en/zend.amf.server.html).
There's no need for AMF though. XML is fine for most cases. And since SilverStripe offers SOAP connectivity, one could also use SOAP instead of AMF (Flash has a SOAP implementation AFAIK).
Hi. I've created a module for Silverstripe implementing ZendAMF as page type. It inherits all the Silverstripe security features as for login levels and access to pages. Adding new features is as simple as AMFPHP. I made a folder where you drop you function, tie connection from ZendAMF to Silverstripe functionality. After that you can create multiple AMF gates and have assign different functions to them + access level.
Example would be one public gate and than access to the rest of gates based on login level. ( gate - page that talks AMF not HTTP )
I wanted to promote this as official module and give that to community to use. But at this stage it is poorly documented and some features I'd like to have are missing. But function wise if works 100%.
If you are interested in this and help me with feedback or some test, or anybody interested in this topic, make a reply in this post and I'll arrange some temporary URL.
I did spent some time putting my module together and make it presentable. And here comes the BUT...
But when I tried to upload it through module/manage/add page I was able to fill all the fields but not able to upload my .tar.gz file. At the bottom of Source tab it says that I'll be able to upload my file after I save the form once. When I save, next it goes to thank you page and there is no chance to upload my files. I do not have SVN or any version control equivalent.
I see you are core guy and most likely I'm bitching to wrong person. :) I'll keep trying and poking that area.