I have heard grumblings about the availability of the Eldar Wave Serpent around the internet ever since the release. This move is no surprise. 

Faeit has the rumor.

On the GW site
Availability: 
This product is expected to despatch in one to two weeks. 
Part Code: 99120104023
 
via Kroothawk on Dakka
I can confirm that from July onward, the Wave Serpent is mail order only. No idea if it will be repacked later. 
Thoughts? Well this is what happens when you mess with stats to sell models. GW made the Wave Serpent just too damn good, not the model is out of stock everywhere.