Turns out there might be a hard -- and permanent -- per device limit to setting up those free Find my iPhone accounts Apple announced alongside iOS 4.2.1, the ones you needed an iPhone 4, iPod touch (2010), or iPad to initiate but could then use on iPhone 3GS and other, older hardware.

TiPb Forum member milani started setting up a few accounts and after the 3rd he was greeted with a popup that said:

Cannot Sign In

The maximum number of free accounts have been activated on this iPhone.

There's a learn more button but apparently whatever landing page is supposed to be there to explain further isn't there yet.

A second popup read:

Account Limit Reached

This device is no longer eligible for creating a free MobileMe account.

Now granted, most users won't need more than one account. They'll create it, set it up on their iOS devices and be done. However some family members may only have one 2010 device to create the accounts on but want one or more accounts for different devices, for privacy's sake. If there is indeed a hard limit of 3 accounts per device it would be neighborly of Apple to "fine print" it somewhere. We looked but couldn't find anything, even in the Knowledge Base FAQ. There doesn't seem to be a way to remove accounts either, so once you've created 3 on a device, that's it, that's all. Forever.

So if you were planning on setting up multiple accounts, be forewarned. And if you've had this problem let us know what, if anything, you've been able to do about it.