Bicycle Mechanics - 9 speed shifter with 8 speed derailer and cassette?

i have a bike with shimano ultegra components. the bike is an 8 speed, but i am replacing the shifters (sti's). if i buy 9 speed shifters (either 105, ultegra, or dura ace) will they work with my current 8 speed derailer and cassette? i find much cheaper and much better deals for 9 speed shifters on ebay than 8. thanks in advance...

No can do. The 8 pulls slightly more cable length with each click than the 9. Replace the 8 speed cassette with a 9 speed and then you're 9 speed all the way. The width of the cassettes are the same so your wheels are cool. The 8 speed rear derailler will work fine as will the front der. 105 cassettes can be had for 40 bucks and a chain for another 20.

Your dreailleurs will work fine. You will need a 9-speed cassette and chain.

The 9-speed shifters will work. You have the choice of buying 9-speed cassette and chain or using the 8-speed cassette and chain by attaching the cable differently at the rear derailer - http://www.sheldonbrown.com/dura-ace.html#indexing

Shimano deraillers are no different between 6, 7, 8, 9 and 10-speed indexing (except for pre-9-speed Dura Ace). 9 and 10-speed chains are narrower. You generally need the same number of cogs as your shifter has speeds for, especially for 8, 9 and 10 (since the cassette is the same width).

If you're going to update, you could just go 10-speed shifters, cassette, and chain. Or 9-speed shifters, cassette and chain.
Or just get new 8-speed shifters if you don't want to update. But your cassette and chain are probably partially worn anyway, so may as well at least to go 9-speed.

Check the regular mailorder channels (Nashbar, Performance, etc.) for prices on 105 9-speed brifters. They are going for either side of $140 since they are obsolete now that 10-speed 105 is available. You may equal or do better than on e-bay and with better confidence in the seller.

thanks...i think i may just upgrade to a 9 speed--another question though, my current 8 speed cassette is 12-23--does the 9 speed cassette i purchase need to be the same?

thanks...i think i may just upgrade to a 9 speed--another question though, my current 8 speed cassette is 12-23--does the 9 speed cassette I purchase need to be the same?
You are going to need a new chain anyway so the cassette can be any range you wish. Common 9-speed cassettes are 12x23 and 12x25. Your rear derailleur should certainly handle either and even a 12x27 if you want to have a significantly lower gear.