Here are my thoughts on this frequently asked question: to summarize though, I would wait if you can, for the simple reason of dollars. Let me explain:
Firstly, you shouldn't have too much longer to wait. Leopard has been planned for release in the first half of 2007. There are only three months of that half left, so the maximum wait *should* be about three months, and possibly less. That is not that long to wait. Admitedly, there are no guarantees that Leopard will be released in that time frame, but it is a good bet. I wouldn't be surprised to see it released any day now. I am rather surprised that it is not out already actually.
Secondly, remember that if you get a machine now with Tiger on it, and you want Leopard when it comes out, you have to buy the upgrade. I am guessing, based on past experience, that this will be about $130. So you will be out an extra $130.00 right off the bat. You also need to consider the time and effort it takes to install the new OS. This is not huge, but depending on your level of comfort with doing this sort of thing, having it come pre loaded could be quite a benefit.
Finally, and this is just a guess on my behalf, I am suspecting that iLife 07 will come along for the ride on new Macs sporting Leopard. This is just a guess, but a reasoned one. If this is the case, waiting for Leopard before buying the new Mac makes quite a bit of sense, since I have heard that, for example, when you buy Tiger, you don't get iLife 06 included - you have to buy it separately. Thus, waiting for Leopard before buying may save you not just the $130 for the OS upgrade, but also the $xx.xx (not sure what the price will be) for the iLife upgrade.
For myself, I am waiting to buy a new MacBook Pro until Leopard comes out, for all these reasons. I realize that this means that I may have to wait for a few months, but I see it as worth it.
In the end, this is a VERY personal call. How badly do you want a new Mac? Getting the Mac now, with Tiger, is not a bad move. We are all of us, by definition, currently using Macs with Tiger on them, and we are all generally very happy with them. There are some perhaps tangible benefits to waiting for Leopard, but these must be weighed against the benefit of having your shiny new Mac right now! As I said, a very personal weighing of priorities is in order I think.