I have to admit, I am a little surprised. The Valve Index limited pre-orders that went live on May 1st have already completely sold out.
Looking at the store page in the UK for the full kit, it's now only showing that you can reserve it with a new expected shipping date of August 31st. However, the Controllers and Base Stations are still showing up as in stock. Oddly the full bundle has a different expected shipping date to the Headset/Controller bundle and Headset by itself with those now expected by July 31st.
It seems for those who are really interested in VR, price is no issue. However, we've no idea how many Valve were keeping in reserve for the initial batch. In the press release Valve originally sent, all they said was that it was "limited". I can imagine them having quite a lot though of course, so it's still a very healthy sign for Valve's newer platform.
Going by the Steam Hardware Survey, the amount of people with a VR set on Steam is currently below 1% of Steam users so perhaps we will see a nice up-tick by the end of the year? For those who keep writing-off VR as a ridiculous niche, do keep in mind the VR market share on Steam is now higher than Linux.