I’ve had a lot of fun using my voice and whistle to play. It works best with tunes that only use to lower octave.
I’ve made a little soundrecording to show what I mean. I believe it’s a tune from a swedish television program called Saltkråkan (based on a book by Astrid Lindgren).
saltkråkan
The whistle is an Alba low D.
//Björn
Ian Anderson … Herbie Mann … and now Mustafva/Björn! 
Sounds great- I’d like to hear more.
You put a smile on my face! Thank you.
Ja, det er en melodi fra Saltkråkan . . .
And I heard it for the first time from you actually! In your recording of Si Bheag, Si Mhor at the Whistle This website.
It’s fun because it gives so many options. You can for example play one note on the whistle and a different one with your voice, like I do the third time in my saltkråkan recording where I begin on G with my voice but play a D on the whistle.
For a beautiful, haunting use of this technique listen to (Rahsaan) Roland Kirk play “Ain’t No Sunshine”. It’s fantastic.
I’ve read that Seamus Ennis use to do something like this while playing the whistle, something about he would hum a note an octave lower and play a phrase or two while doing so… I can’t for the life of me make that techinque work myself. 
-Eric
Yes, that was a nice one, funny beginning also . I found it at youtube in this clip: http://www.youtube.com/watch?v=-RuY4_AFmHs
pretty nifty, that!
That Seumas Ennis whistle thing is very interesting as it harks back to what Tomas O Canainn writes of sean nos singing in “Traditional Music In Ireland” : “Nasalisation, as practised by traditional singers, seems yet another attempt to maintain continuty musically by continuing a note at the end of a line even when there is no text to support it. It is most obvious on a vowel sound when the singer closes his lips thereby forming an “m” sound like a kind of drone which is repeated at the end of other lines. A drone accompaniment is so natural in this music that the practice is quite accpetable…”
I puzzled over this, wondering what it might sound like. Then, at a quite unexpected time and place, I heard it in person. During Mass one day I happened to be quite close to our old Irish-raised priest (a fluent Irish speaker and great singer) while he was chanting. There was that drone, coming I presume from his nasal cavity. It was fantastic.
But, this of course is quite different from the Roland Kirk flute thing, where you are humming in unison with the tune you are fingering on the flute, and the two blend perfectly (if you hum in tune!) unifying to create a new timbre, not voice, not flute, but a new sound.