Semi-tropical plants used as house plants can often handle temps into the low '40's in the fall as temps drops gradually. Those same plants that have been kept warm all winter indoors, struggle when exposed to suddenly much colder temperatures when moved outdoors in spring. Just as plants must be acclimated gradually to changes in light, they must also be acclimated to temperature changes.
Personally, I prefer to keep my houseplants indoors year round. Moving plants in and out each year causes a certain amount of stress twice every year. In addition, there is the problem of importing pests indoors in the fall. In nature, plants put down roots and stay in the same place throughout their lives - something to consider.
