There's a chance with every piece of RAM that it may not work correctly. Personally I think the problem was with the early machines being manufactured. It should be fine now. Where are you buying it from? You should always buy from somewhere with a good lifetime warranty and a good return policy.
OWC has a good price on their RAM plus a lifetime warranty. $59.99 for 2X2GB.
No matter which mfgr you buy the RAM from, it may not work. Sometimes it's just DOA. If it's the right RAM, a reputable mfgr will replace it. Stick to name brands.
I just installed the 4GB kit that flopticalcube linked to above earlier this week into my 2.4GHz UMB and it's fine, no problems at all. If you do install it and experience problems it's just faulty RAM, not compatibility issues.