I bought an Acer laptop a couple of years ago just before Vista came out. It touted a free upgrade to Vista because Vista was imminent. Fair enough I thought, I'll still get Vista and don't have to wait so I went ahead with the purchase.
The real story behind the free upgrade was that (1) I had to wait for months after Vista had shipped and was already available on a similar model laptop at Best Buy and, (2) I had to fill out an online form and then mail in all sorts of proof of purchase and, (3) I had to pay $15 for shipping and handling. This was a big effort, wait and annoying cost and that's the last time I fall for that free upgrade story.
Interestingly I just read Free Windows 7 upgades not always free and it reminded me of my Vista saga. However it looks like Acer may have learned from the Vista experience as they are now the only company listed in that article not to charge for the Windows 7 upgrade. However, I still wouldn't buy a computer with an upgrade promise because I can guarantee that you will have to spend hours fighting for that upgrade and you will only get it long after it's become available on the same laptop in that same store that you bought your Vista machine in.