I do not see the 986 as an investment, considering the maintenance costs it would be the worst investment I have ever made. If you are buying a mass produced car as an investment you really need to reconsider. That said, I do see 986 prices stabilizing and even rising.
I would say 986 prices bottomed a few years ago, and have been climbing very, very, very slowly since. I think this is likely because as the original and 2nd owners have unloaded them in the sub-$10k range, the buyers in the sub-$10k range are balking when hit with the high maintenance costs (which is completely reasonable when you compare the maintenance cost to the value of the car). This leads to a lot of 986's being taken off the road, which thins the supply, pushing prices up, very, very slowly.
This is nothing new, and happens to almost all Porsches at some point. Just look at the used 911 market over the last several years. Or the 944, or the 914 over the last 30 years. If you still have a good condition, low mileage 986 in 30 years it might be worth something, but you will have probably spent five times the profit on maintenance by then.
@ Luv2Box, all Porsches get classified as 'classic' after a certain number of years. It just means the price of spare parts goes up, as they produce smaller batches less often.