Questions about NFC's card emulation mode
After quite some research I figured it out myself and wanted to share my findings with you.

NFC card emulation types

Please bear in mind that NFC is still not a finished protocoll and changes are possible. This is just the list of what I found as the latest version. (Copied from C. Enrique Ortizes' blog on January 10, 2011)


NFC Forum Type 1 Tag Operation
Specification – Type 1 tag is based
on ISO14443A. Tags are read and
re-write capable; users can configure
the tag to become read-only. Memory
availability is 96 bytes and
expandable to 2 kbyte; communication
speed is 106 kbit/s.
NFC Forum Type 2 Tag Operation
Specification – Type 2 tag is based
on ISO14443A. Tags are read and
re-write capable; users can configure
the tag to become read-only. Memory
availability is 48 bytes and
expandable to 2 kbyte; communication 
speed is 106 kbit/s.
NFC Forum Type 3 Tag Operation
Specification – Type 3 tag is based
on the Japanese Industrial Standard
(JIS) X 6319-4, also known as FeliCa.
Tags are pre-configured at
manufacture to be either read and
re-writable, or read-only. Memory
availability is variable, theoretical
memory limit is 1MByte per service;
communication speed is 212 kbit/s or 
424 kbit/s.
NFC Forum Type 4 Tag Operation
Specification – Type 4 tag is fully
compatible with ISO14443A and B
standards. Tags are pre-configured at
manufacture to be either read and
re-writable, or read-only. Memory
availability is variable, up to 32
KBytes per service; communication
speed is up to 424 kbit/s.


NFC card emulation technology

Basicly a RFID card is emulated by a Java MIDlet using Contactless API (JSR 257). That is able to access the place (either SIM card, a smart card, a secure internal memory, or a secure external memory) where the emulation information is saved.

The exact how and wheres of NFC card emulation seem to be a matter of intense discussion right now as there are two basic concepts that directly correspondent with my second question (provider vs manufracture (vs user)). That this discussion makes a right answer very hard and fundamental changes can also happen is best shown by this example: Sun's official developer introduction to NFC uses an older copy of a graph from C. Enrique Ortiz to explain how card emulation mode works, that now is completly outdated and wrong.

The GSMA pushes for the simcard to be the storage of the smart card information giving the provider total controll over NFC card emulation. I personally would favour a solution among the lines of "make everything possible".

Programming Approaches

While searching for programming examples I found out that with the currently available NFC enabled devices (mainly from Nokia) that there is one always stated advice: One MIDlet per RFID card issuer (Assuming that cards from the same issuer are from the same type and work alike).

Since there is no final decision on where to store the smart card information, implementations go as far as having a download manager to download various smartcards to emulate over the internet.

Manufracture / Provider limitations

It seems possible that a manufracture can implement as much NFC functionality as he wants. Nexus S, if not updated to Android 2.3.3, for example has has only TAG reading functionality.

If it would be decided that the sim card should be the only storage area for card information, space limitations can become a problem, especially if you want to emuluate several cards.

Conclusion

This has been just a quick summary of what I found out and will probably be wrong because of NFC protocoll changes in the future, but for now I hope it helps some one that askes the same questions as me.