Ah, life in the US - really, the FBI could kick down my door and take all my computing equipment for any or no reason at any time. Grumbling about Twitter or Amazon is about as controversial as I get, though. Is it better to have your data stored in 'the cloud'? Not really. It's vital have encrypted backups here and there one way or the other.
The door kicking and the border scenario are different. With the door kicking you had to first be noticed by the FBI, but with the border crossing you're placing yourself under the gov's arbitrary notice. Both scenarios are unlikely. Unless you're controversial, or you happen to get a border guard on your or their bad day.
I know police have been looking through mobile phones at traffic stops for a few years at least, too, and now some even have device which will copy the entire contents of the phone for later analysis. Traffic stops can be pretty casual.
I'm wondering, though, are you saying I should be concerned about losing the hardware or the data in particular? You mean, store everything in the cloud so access to my hardware doesn't automatically mean access to my data? How about TrueCrypt or Apple's encrypted home partition option?
Definitely. An even more likely scenario for someone as fritter-headed as I can be is that I'll leave it at a coffee shop or hotel, no outside malice required! Definitely a big concern for mobile devices, and I've been thinking lately about how crucial this makes my MBP. It's definitely not as safe as the system at my house.
What I used to do is have everything on my home system, then connect to it through VNC or NX from the road. It's great when there's a fast connection available, but too frequently VNC is unusable due to unreliable internet connections. I'll get this network thing figured out some day.