Sadly, as a former Starbucks barista, I know that the pumpkin spice latte is little more than pumpkin-flavored condensed milk syrup, loads of milk, espresso, and VERY sugary whipped cream (they do use heavy whipping cream, however).
Now, I've had some success with flavoring my coffee by mixing about a tsp of cardamon and cinnamon in my coffee grounds prior to brewing. You could probably do the same with a pumpkin spice mix in the grounds. Then, it's just warmed/steamed milk/cream (you can froth it on the stove, too!) and a dash of pumpkin spice mix or cinnamon for garnish.
There's an intriguing Pumpkin Whipped Cream recipe over at Sandra's Easy Cooking blog that could bring a more realistic pumpkin-topping to your concoction. Caution: I've never tried it. But, it appears simple enough!
Can you tell that I'm a seasoned coffee-slinger? THIS IS WHAT I DO.