Having recently spoken about this topic to a long time certified GM tech I was told that GM 2.8, 3.1, & 3.4 V6 equipped car owners could expect to have to replace the lower intake manifold gaskets on a regular maintenance schedule of somewhere between every 50 to 125 thousand miles.
The problem is one of different expansion rate characteristics between the cast iron cylinder block and the aluminum cylinder heads and intake manifold sections. As the individual components heat up and cool down over many driving cycles this literally wears out the gaskets between the surfaces even if the very best silicone sealers and loc-tite products are used on the sealing surfaces and corresponding mounting hardware.
If the engine coolant on your car suddenly seems to be going somewhere but there are no signs of external leaks, check the oil level on the engine oil dip stick for a higher than usual level or an unusual muddy or foamy look to the oil itself. If any of these symptoms are suspect do not continue to drive the car until it is properly tested and diagnosed. There are many reports of catastrophic engine failure due to coolant being picked up by the engine oil pump causing severe bearing damage and in some cases even causing camshafts to break. This is possible because the antifreeze and water mixture is actually heavier than motor oil and will settle to the very bottom of an oil pan completely displacing the motor oil that would normally be circulated to all of the engine bearing surfaces, especially during a cold engine start up.
Check out the following links...
http://www.autosafety.org/uploads/SB-10001282-8175.pdf