As promised:
Please understand that I mean NO disrespect for any type of Port wine, but will only prognosticate on Vintage Port pricing; which is the specific category that is not only the most marketable, but also the most widely collected type of Port, outside of Portugal.
First let's have a look at the fundamentals ... supply and demand. Afterwards, I'll provide some insight as to what I foresee as a successful buying strategy (of course your mileage may vary ... but this should open up some lively

discussions )
Worldwide supply and availability of young to almost mature Vintage Port is strong at the moment and there is no impending shortage in sight given purchases this decade so far. That is a good sign for short term pricing of young VPs.
OTOH, there is a dual reason to be concerned about older bottlings of Vintage Port. There is a growing scarcity for bottles older than the 1970 vintage and far more so for the important names bottled prior to 1950.
Not so unlike Madeira, the resources for these mature VPs is slowly but surely becoming more difficult to access. Vintage Port has never been a simple commodity, even thought it has always been very marketable. Nonetheless, the small niche which it enjoys is filled with enthusiasts young and old. Visit Oporto, Vila Nova de Gaia and/or the Douro and you will see many more fairly young Port drinkers and tourists.
The other main reason why it continues to grow harder to find Vintage Ports from the 1970s and older is that numerous Port shippers make a concerted effort to recoup their own stocks quietly. This is no secret, but also not something often mentioned.
This combined dynamic assures that in time, the older bottle prices will surge upward at a moderate pace for up to another decade and then I believe they will become ever more scarce and pricey. This will magnetically bring along noticeable increases in Vintage Ports from the 1980s and better VPs from the 1990s too.
Of course there is no reason to panic by any stretch of the imagination, but the increases will be along and will appear similar in degree to what was felt by buyers during the mid-1980s compared to the mid-1990s. And that is for the VPs from the past 20-35 years. Again older bottles will cost a pretty penny and make today's pricing look like a bargain.
In perspective, I paid $19 for the best of my 1983s. Top 1994s were $38-40 on pre-release and less than 15 years ago, 1977 Fonseca and Taylor (2 benchmark VPs for pricing comparisons) were easy to find for under $50 per.
Now lets get back to the near term buyer and what successful buying strategy should be employed in building a solid collection of Vintage Port (and here I also encourage the pursuit of both Colheita style and Tawny Port with-an-indication-of-age).
The Euro's strength versus the dollar has many Port importers, (here I speak specifically of the USA) "puckered" for lack of a better single word. It is my opinion that the Euro will continue its rise and we may see values of nearly 1.5 versus the dollar (currently resting at 1.42) by the end of 2007. Unless something happens dramatically to change events, further slippage of the once mighty buck, is more than a possibility. Regardless of the outcome of our next Presidential election, that is the earliest that I can see any real devaluation of the Euro vs the USD.
US Port importers have very little current inventory of Vintage Port at significantly lower Euro pricing. Many who fortuitously took positions in the past have depleted those hedged resources. Look for pricing in 2008 to remain within a few % points of today's pricing unless the 1.60 Euro becomes a reality within the first half of the year. That can be taken as a judicious "buy" signal if I was not clear.
Although there are stocks of the 2000 and 2003 Vintage Ports still on shelves and even being discounted, all of that could quickly change when in 2009 there will not only be a change of Presidents in the USA, but if there is a generally declared 2007 vintage release ... the two other vintages from the current decade could finally start to show signs of strength.
Between now and this time next year: buy smart by grabbing the blue chips from 1980 through 1994. With whatever funds are still available for your Port passion, back fill with the gems from 1963 through 1977 before they too become not only harder to find in perfect condition, but at noticeably higher prices.
Five year forecast: Pick and choose your older bottles carefully and possibly in half or full case lots (if from known provenance) or dollar-cost-average your specific collectables by purchasing 2-3 bottles at a time over the next 15 months through 2012. It will be wiser to shoot for the older bottles rather than stocking up on younger vintages which will still be relatively priced given where the market is today.
This is my view of pricing and stategems based on conversations with the Port trade, my own reading and past history as a consumer and advocate for VP. Obviously I have no crystal ball and only claim that my opinion is just that. Take it for what it is worth to you.