We're in the same boat -- I think we've brought in a dozen or so since the initial release of the iPad Wi-Fi version.
I initially listed them as Computer assets but they quickly got lost in the mix. I wound up creating a new resource type with a base type of Computer, but excluded the network resource details, power consumption and rack units dataclasses since they didn't really apply. This way we can still record the cost info, serial number, ownership, etc.
Since there's no agent for them, they're treated similar to monitor assets and it's up to process to ensure the inventory remains accurate on them.
One of the things I liked the most about making iPads their own data class is that I tweaked the default values of the asset page so my data entry efforts were minimal when keying in new assets (depreciation/warranty info set appropriately, Manufacturer already listed as Apple, etc.)
Regarding the iPhone subject, we did something similar there as well. We created a new resource type (Smartphone -- since we have iPhones, Blackberries, and Androids in our environment) with a base resource type of Cell Phone. We have a Smartphone data class tied to this resource type which captures data like the ESN, service provider, etc.
Hope this helps!