Suggesting the Cucuteni-Tripolye as the source of PIE. Something I did wonder once after seeing a pretty old wheeled toy from that area. They were the most advanced civilisation (not too strong a word, they had small cities) of Neolithic Europe, and were one of the first cultures to use metal.
Cucuteni-Trypillian cow-on-wheels, 3950-3650 B.C
One of the more interesting points from it was that word for wheel you find in other languages seems to have a root in the PIE word to turn/rotate. As far as I know, the worlds oldest wheel is 5,300 BP, dragged up from a Slovenian Marsh.
Jim Mallory (1989: 163), on the other hand, goes a long way towards the here proposed solutionwith the following observations:
“Tomas Gamkrelidze and Vyach[e]slav Ivanov… have noted that … Proto-Indo-European *kwekwlo- bears striking similarity to the words for vehicles in Sumerian gigir, Semitic *galgal-, and Kartvelian *grgar. With the putative origin of wheeled vehicles set variously to Pontic-Caspian, Transcasucasia or to Sumer, we may be witnessing the original word for a wheeled vehicle in four different language families. Furthermore, as the Proto-Indo-European form is built on an Indo-European verbal root *kwel- ‘to turn, to twist’, it is unlikely that the Indo-Europeans borrowed their word from one of the other languages. This need not, of course, indicate that the Indo-Europeans invented wheeled vehicles, but it might suggest that they were in some form of contact relation with these Near Eastern languages in the fourth millennium BC.”
Since the Trypillians weren’t that far at all from the steppes area, I can see this might have some validity. The Dniester site is just in my ‘had wheels’ at the right time zone, and the timing isn’t massively far off. This might allow a compromise between the 9,000 BP ‘first farmers’ and 5,500 BP ‘Kurgan’ theory, as they probably did speak the languge of the expanding farmers; that part of the world had a respectable demic wave from Turkey appear in it.
Mathilda's Anthropology Blog. 
Is there a wheel found at
http://en.wikipedia.org/wiki/Mehrgarh
!!!
Is there a wheel found at Mehrgarh?
Don’t think so, it’s just outside the wheel era for that area at 3200 BC (terminal), the Indo Euroeans appeared with the wheel as their culture disintegrated.
Read the letter (not really any research paper here) and find it just a piece of speculative tought.
While I won’t contest that Tripolyans might have used wheeled vehicles (just like other cultures did as well), the author describes them gratuitously as proto-IE, while providing absolutely no basis for that claim.
CT culture was a Danubian branch on native Neolithic substrate (surely of Epigravettian origins). They appear to have been rather advance and original and rather related with their southern neighbours of Boian-Maritza and Karanovo-Gumelnitsa, even more advanced.
In their late period they were subject to very intense IE (Kurgan) pressure apparently, who seem to have plundered the country continuously but who only conquered it at a relatively late date (they apparently had more succulent preys elsewhere). In fact Bronze Age Foltesti culture of Moldavia, directly derived from CT, may well have been the last surviving Danubian culture of all Europe.
In any case, CT is not the archaeoogical ancestor of any other culture, excepting arguably Boian and its derivates in historical Thrace. Hence talking of it as “PIE” makes absolutely no sense: if CT was the PIE urheimat, nobody would speak IE nowadays (they just went extinct as culture eventually and expanded to nowhere).