Recording JavaScript actions / ASP.NET Postbacks

Do I need to do anything special to record a Javascript JSON request in a Web Test?
Particularly, ASP.NET Postbacks don't seem to be working for me.

Windows7 Ultimate (64bit)
SoapUI Pro 4.0.0 Trial (32bit)
Firefox 6.0.1 installed

Should I be hand-creating these requests?
Anyone know of any good tips / tricks / articles / screencasts that explain this?

Thanks,