I'm planning on buy a T4xx/T5xx around September.

The other day Microsoft were quoted as saying that Windows 8 will be released 'in a couple of months'. I don't really want Windows 8 on my new machine so I'd like to make sure I'd ordered in time to still be able to specify Windows 7.

When a new release of a Microsoft OS becomes available how soon after that does it become impossible to order a Thinkpad with the previous version of the OS ? Is it more or less immediate or is there a period of weeks/months when you can still get the old version ?

Even today you can still buy XP.
W7 will be available for quite a while yet (several years).
If W8 turns out to be the abomination it is supposed to be, M$ will start developing W9 in a hurry.
Either way, you buy the laptop from Lenovo, who can put on what Lenovo (or the buyer) wants, NOT what M$ wants!

Wow ! OK I didn't know that. The only thing is when I go to build a quote on my (non USA - http://shopap.lenovo.com/nz/en/products/laptops/thinkpad/) version of lenovo.com I get offered various flavours of Windows 7 but I don't, for instance, get offered, Windows XP .

Maybe this is something you have to do on the phone to a sales rep ?

You can still buy XP on the open market, but not any more from PC/Laptop providers.
Sorry if my info was incomplete.
XP will lose support from M$ in 2014.
For this and similar info on other OSes, see this: http://www.zdnet.com/blog/bott/how-long ... ows-7/2304

Well FWIW and for future reference I just did the online chat thing via the lenovo site and the response to my question was : "we don't have any information on that" (not quite sure if I was talking to a bot so maybe this was just the bot falling out of a CASE statement)

Then I got on the phone to Aus/NZ sales and the sales rep says "as soon as Windows 8 is out there Windows 7 will no longer be an option" ... sigh.

I seriously doubt that based on past experiences with Lenovo...W7 will be around for quite some time.