Steve,
That's a great idea! This method would also keep your backup battery bank from developing memory. Sooner or later, the price of everything always goes up... the best time to buy something durable is now, give or take a price adjustment. Your dollar is always going to be worth less in five years.
Djimm,
I've found a webpage that describes the type of system you inquired about. Literally jam-packed with examples, pictures, pricing, and lots of pertinent reading.
Emergency Solar Backup Power, AC Coupling, Home Generator Kits
The main difference between a standard, grid-tied system and what you ask about is what happens when you have a power outage. With a standard grid-tied system, the panels automatically shut down when utility power is lost. This is done to protect firemen, electrical linemen, and other workers from being electrocuted by your solar panels. A permit is nearly always required to tie anything to the utility lines that can generate its own power to ensure the safety and ability to disconnect the secondary source of power.
With a Backup system, the utility lines are disconnected from the solar generator the same way, with a plainly recognizable "smart" disconnect for safety, but part of the system remains energized. The back-up power generation/batteries/inverter can be rigged different ways, with different components, for different needs or modes of operation. The main idea is: do your homework and get the right components that will operate how you want them to. Cuz they tend to be expensive and sensitive.