do all older iPhones screw up on ios 4?

There is a small amount of phones that have had problems..I think the majority have updated firmware and never have a problem. All of my phones have been updated, no problems.. I thought my wifi had been affected but a simple open and stick it back on again worked. If you don't want to update then don't but the only way to downgrade is to jailbreak or use tinyumbrella if you've saved your shsh blobs..Jailbreaking has it's own little pitfalls, and as you are on ios 4.x you may find that the only one available to you is the ipad jailbreak which may wreck gps/wifi anyway!

Sometimes I think that people find coincidence and link it to problems, bit like someone who tries lobster mornay for the first time and gets food poisoning, the fact that it was the pate earlier in the day doesn't seem to register..

Dropping phones breaks speakers, the 3GS is 2 years old-the battery is at the end of it's life and is bound to start failing. Maybe the update uses more energy for some processes and this hastens the battery death? Once a battery starts failing they go very rapidly downhill. People link these things and blame ios updates.

If you are truly worried sit back, leave the phone as is and wait until everyone else has tried it and reported any problems. imho no problems so far..