Right now, you only have two options to sync bookmarks on the iPhone: plug it into iTunes, or use MobileMe. Paying $100 a year for MobileMe access to my bookmarks seems a little excessive, and I find that I'm not plugging into my mac often enough to really bother with bookmark syncing. Xmarks offers a coolservice to sync bookmarks across computers, but Apple won't let anyone else sync bookmark data (Xmarks is read only). Either a standalone app or, ideally, a plugin that emulates MobileMe and automatically syncs bookmarks between the Xmarks server and the iPhone would be very cool, and it is something I would be willing to pay for (i.e. more than a buck!).
I've looked to see if Xmarks has released their API's yet, and they haven't. Not sure if you can reverse engineer it, or if the API's are absolutely required for the development of Xmarks-like functionality. I just can't get over the hurdle of forking over $100 for a data transfer; it would be like paying $100 to open a web page, or take a phone call.
Anyone want to take up the gauntlet? I use MobileMe bookmark sync, but I wouldn't mind dropping it in favor of either Google Chrome Bookmark sync (via Google Docs) or Xmarks sync.