I recently made an adaptation of the SoftwareSerial library for Teensy 3.0. It does not actually implement software-based serial, but if you give it the pins for real hardware serial, it allows you to use libraries and sketches that depend on SoftwareSerial. All you have to do is edit the pin numbers in the SoftwareSerial instance to match the pins for a real serial port.
Here's the code:http://www.pjrc.com/teensy/beta/SoftwareSerialReal_test1.zip
Well, that's all you have to do for Teensy 3.0. This can be adapted pretty easily to Due. You'll have to change the #if defined(__MK20DX128__) to __SAM3X8E__, and in the constructor you'll have to edit the pin numbers to the pins Due uses for those ports. But those simple edits should make it work.
There are a good number of libraries and sketches with hard-coded dependency on SoftwareSerial, so this code should help Due users.... so I'm sharing it with you. Christian, if you're reading this, feel free to include it in 1.5.3 if you like.