Squark wrote:
> I have recently learnt about the は+が construction in Japanese, such
> as in the sentence 男の子は目が青い (the boy's eyes are blue).
> The question is, what is the difference between this and の+は, for
> instance 男の子の目は青い.

The difference is 'the topic' of the sentence, in other words, 'what you 
are talking about'. Particle は indicate the topic of the sentence 
different from が that indicates the subject.

For example, in the sentence 男の子は目が青い, you are talking about 
'the boy' whereas in the sentence 男の子の目は青い, you are talking 
about 'the boy's eyes'.

Practically there is not much difference in the meaning of two 
individual sentences. However, indicating the right topic is as 
important as indicating the right subject in the Japanese discourse 
because by detecting the topic, you can guess what will follow in the 
sentence, and as a consequence, you can save your energy to comprehend 
the whole context.

> are there some cases where the は+が construction is
> applicable and the の+は isn't?

Yes. わたしはあなたがすきです is the example.

萩原＠グリフィス大学