If you have an original iPhone (the Edge only one), it came with a dock. This dock fits the new iPhone 4 and still charges it and seems to hold it pretty nicely. So if you were looking at buying the new dock and still have one of these older ones around, try it out first.
[crarko adds: I haven't tested this one. Given the difference in thickness between the two units this is a lucky find.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100625080420633