I'm not sure why the flowers have spots on them, usually this is caused by a fungus in conditions where the air isn't moving and it's is very humid. Or perhaps you are misting them when you water? In that case, try to avoid getting water on the flowers.

The flowers may be wilting just because it's time for them to go, it sounds like you've had this plant for a while. I would also discourage the use of fertilizers while the plant is in bloom, some people believe this causes flowers to die earlier.

As to the spot, this is most likely either a fungal or bacterial infection. Take a pair of clean scissors and cut the leaf at least 1/2 or 1 inch (depending on how fast it has been spreading) around the spot. Make sure you don't touch the scissors to the spot as this may transfer the infection. Usually spots like this are caused by conditions that are not optimal for the orchid.

Some additional questions to help us with culture recommendations:
How much sun is your plant getting?
How humid is it where you are?
What are the day and night temperatures?

Please also refer to phal culture sheets by the Canadian Orchid Congress. These can help you fine tune your conditions so the orchid will bloom again for you soon.
~Jem

I am not an orchid expert by any means.... BUT (and this is a big BUT here)... usually the blooms (of any plant) that have spots on them are due to botrytis. That is often caused by excessive humidity and water on the blooms, sometimes combined with cooler or cold temps, exactly what jemsta mentioned.

Just as a side bar - botrytis is not always a bad thing, only when blooms are important. Botrytised wine (aka Ice Wine, Late Harvest Wine, Frost Wine, etc) are very sweet and delightful because the grapes have been on the vine so long the cool temps, and morning humidity or frost has caused the botrytis fungus to flourish and it has begun to break down the fruit and sugar is very concentrated. If you like really sweet (honey-ish) types of wine - give one of these a try.

Alexa