Good question I feel the answer is yes for most as apple updated the iPhone, iPod touch and nano last year changing the charging method At that time some older accessories would not charge. I believe if your accessory works and charges the iPhone 3GS then the new iPhone 4 should be fine as I am unaware of any hardware changes at this time. In saying that I own a parrot MKi9200 in car system which I use for my iPhone 3GS Fully integrated hands free with separate control over playlists. This product is a little touchy with upgrades and may not work 100% with iPhone 4 until they issue an upgrade. I will write a review on then Parrot once we actually can get the iPhone 4 in Australia which is around late July at this time. I suggest if considering purchasing any accessory that you try the iPhone 4 in the unit before parting with your money
I think we are going to have to wait until the companies say the products will work with the new iPhones.
I have a Memorex alarm clock radio with iPod dock on top and it works with the iPod Touch and 5th Gen iPod just fine but doesn't work with my Brother's iPhone 3G. It will charge it but doesn't play music from it.
Many of the sites I visited specify which accessories work with the iphone 4 and cases seem to be labeled for which phone they are for but haven't seen that labeling for other items yet. Guess its wait and see.
regardless whether you have iphone 3g or iphone 4 or any other iphone, you may find these iphone 3gs accessories(here at Fones.com) applicable on your iphone. There you can find many applicable iphone accessories coz I have an iphone 3gs but cases and other stuff can well be fitted and applicable on other iphone models.
Iphone 4 has the same multifunction headphones which are included in the previous models, so nothing to worry about the accessories it is the case which cant be used with the Iphone 4, because Iphone 4 is thinner then the previous ones.