Currently I'm primarily Mac-less, since I'm in limbo waiting for a new iMac to be shipped. That means I'm relegated to using my MacBook, which, while I love it, is just not my primary development and all-around-life machine. I do pretty well in terms of mobility- my email is pulled via IMAP so all of my email, folders, and message read settings are constantly up-to-date whether I'm on any of my Macs, iPhone, or web browser, but there are some things that I still find missing. Namely, iCal and Address Book.
The "official" way to do this is via .Mac or other pseudo-.Mac service. But, while I really could use that seamless up-to-dateness, .Mac, at this point in time, simply doesn't add a lot of benefit for me for how much I'd have to pay for it. This is a common complain for .Mac, really, and I'm not the only one noticing this. I've been hoping for Apple to revamp .Mac to make it more useful (or drop the price on its current offerings), but they're just not quite there yet.
So as a hint to iPhone users out there: I've found that the iPhone's multi-sync capability works well in this regard, particularly if you don't necessarily need up-to-date changes pushed to all of your Macs. Just connect your iPhone to your second Mac, then you can go into the "info" tab and check off what you'd like synced on the new machine. When you press "sync", iTunes will give you the option on whether you'd like to wipe your phone with data from your second Mac, or if you'd like to merge your details between your phone and your Mac (which is what you'd probably like). Sync it up, approve any iSync conflicts, and you're good to go.