How to effectively UV unwrap a subdivided mesh? Are problems usual to arise in unwrapping a subsurfed mesh? Specifically in my case, I have a mattress that I have modelled for a bed, when I try to unwrap this the first problem that arises is the message that pops up in the info header,,
first problem:  what does the message mean?   

The UV layout that I have created after hours of untangling, is(the green box here marks the area with the only pattern in the entire texture) :
,
second problem: was this a proper way to uv unwrap the mesh. 

Furthermore, I had created an experimental texture in GIMP which I applied to the mesh:

but the render turns out a bit stretched:

What are all these problems? How can I fix them? What is the correct way to uv unwrap and texture a subdivided mesh?  

I have also included the file; with the texture packed:
The message is because the scales of your object are non uniform. That means that some axis has a different scale than the other (eg : (1, 2, 1)).

But, unwrap functions works on uniform scale. So, in order to keep the dimensions and unwrap with the good proportions, you need to apply the scale (with Ctrl+A then 'scale').

Here you can see the impact on the unwrapping :



By the way, it seems that your initial work on the UV map give few space for the main face of the mesh (it is in a small center part of the UVs).
You should unwrap the main faces as 'big surfaces' in order to have the best benefit of the texture.

Something like that, which is obtained automatically adding seams to each corner :