No you cannot
The overall OS limitation hasn't been changed in 4.0, what really has changed is internal request for different operation of the OS.
For instance if you want your Application to still keep alive your Audio capability you have to write a module in your App and define it as the part that links streaming audio to headphone, this part is kept alive with simple idea of link on one side and output which is Handled on other side by the OS, your actual App is closed anyways.
Quick App swapping is nothing more than saving the state of your app and closing it, when your app is started again it opens normally but its saved state is reloaded from the EPROM, thus it doesn't use any extra memory or processing to handle these "background" apps.
Even iPhone 1.0 was multi tasking capable its running FreeBSD in the background anyways, but these features are limited by iPhone's Framework, thus still Application's can't be built that could blacklist incoming calls as they do not have access to baseband unless you jailbreak it.