I just purchased last summer in Park Terrace, mission bay. I didn't have the hardwood floors put in but I will be doing so before I sell. My carpet is getting wear and tear now but I will be upgrading to hardwood for sure before I sell. I recently had my place reappraised and the appraised value was up significantly over the past 6 months.
My advice is: don't do it just for the investment aspect at this point. If you WANT the upgrade, go ahead now.
The benefit is a mixture between your personal enjoyment and investment. If your unit is highly luxurious, there is a deep market for such units in San Francisco. It's the starter properties and lower priced ones that are being hit by the subprime crisis.
Other than simply wanting the upgrade, it depends on how long you've been in the unit, when you plan to sell and what the market is like. If you would like me to send you market information, let me know the address and unit, I will send you a market analysis. Bwilliams@ubayp.com or give me a call and I'd be happy to discuss it with you.