iBooks, Kindle and Kobo support that, provided you download the ebook(s) on all devices you're using.
I'm using iBooks on both my iPad and iPhone. At home, I read on my iPad; on the road I'm using my iPhone. Synching reading positions, bookmarks, highlights, etc. works just fine (automatically) over both WiFi and cellular networks and doesn't cause a lot of data traffic.
Kindle only synchronises manually, AFAIK, and only the *latest* position. I don't like that.
Kobo is out of the question for me for serious usage since it *requires* registration to do anything at all. Plus: I really don't like its look. Synchronising worked for me when I tested it once (for the article in my signature).