Being relatively new to whistles and the Board, I may have missed previous discussions on the subject (so I count on your forgiveness). I was wondering about tunable vs. non-tunable whistles.
All my acquisitions so far have been non-tunable. Since I really didn’t know much on the subject, I only assumed (bad!.. I know) that non-tunables would be more stable, and that the price of whistles being easier than let’s say… guitars (my other addiction), I could always buy them in different keys.
Then, I realized that, maybe, tunable only meant the ability to “fine-tune” a given key, and not give the option to play n other keys.
Wow, that sounds confused, doesn’t it? Well I guess I AM confused a bit. Anyone willing to provide a primer on the subject?
If you only play alone, you don’t need tunable whistles.
Tunable refers to being tunable within the same key.
But if you play with another person or a group, you will probably need a tunable whistle to avoid having the entire group to have to tune to you. If there are 2 non-tunable instruments, then there will be a problem. It’s always a good idea to be tunable, but even when whistles are tunable, they’re not tunable by that much.
I remember a recent thread called ‘Is Tunable Essential’ or something close to that. You might want to search for it. 
Tuneable does indeed only mean fine tuning within a given Key.
To add to Cran’s answer, most cheap whistles (plastic head, metal body like Generations) can be finetuned by loosening the glue holding the head (with hot water) and moving the head.
I always buy tuneable where possible because room temperature and altitude can also affect pitch slightly, and even playing along to CDs you might need to retune the whistle.
Here is a link to the thread that Cran is talking about.
All the Best, Tom
http://chiffboard.mati.ca/viewtopic.php?t=10635&highlight=tunable
I have found this to be basically true. However, I have a Dixon G and a Dixon A, both tunable, and the tuning slides will pull out far enough to get a decent F#/Gb and G#/Ab with a little creative breath pressure. They will not go sharp, though.
Brassblower
To “sharpen” the scale of a whistle to the next “key” you can employ an old Bagpipers trick, insert a straw(drinking)up the bore with the top end pinched/glued shut. This will decrease the internal cubic area enough to play in a sharper key. Trial and error will give the correct length to insert but I suggest a semi-narrow straw.
There will be a slight problem with the lowest and highest notes with a straw inserted because the tonehole spacing will be “off”, but embouchure “bending” can cure this.This technique can be used to sharpen “Non-tunable” whistles too.
Reducing the diameter of the bore end hole with beeswax or sticky putty can lower the key about a half tone or so, but the high notes in the 2nd register will need embouchure bending to keep them from going sharp.
Way kewl, Thomas! 
However, through the magic that is known as WhOA, I have a full set of Gens, a few other D whistles, a Susato Low F, and Dixon G and A whistles. Thus, by pulling out the slides on the Dixons, I can reach all 12 keys!
Still, the D and G whistles get the biggest workout. However, I am starting to use the Dixon A in place of a D whistle more because: 1) It will hit those low B and C# notes that sometimes show up, 2) I can play it in the key of E, and 3) it’s LOUD!