As they're the only two devices right now that really use the smaller microSIM format, it comes as a shame to hear Apple confirm that swapping the cards between the iPhone 4 and iPad WiFi + 3G won't be possible. According to the Apple UK FAQ for the iPhone 4, while the smartphone is provided - in the UK - unlocked and SIM-free, "your iPad microSIM card will not work with your iPhone."
The official confirmation puts pay to the idea of having a single iPhone 4 contract - including a data plan - and then switching the microSIM between the smartphone and the 3G-enabled tablet rather than paying for separate iPad data. Of course, given the UK carriers seem to be clamping down on mobile data allowances in the run up to the iPhone 4's launch, it's not like you'd be getting through much content on 1GB of data or below, but US iPad 3G owners who jumped onto the AT&T true-unlimited data package may be disappointed not to be able to use that microSIM in their iPhone 4.
iPhone 4 hands-on:
[Thanks to everyone who sent this in!]