I use iPad 1 and 2, Windows laptops, Windows netbooks, an Android phone, an iPhone and a touch 4, a Kindle Fire, and I've got my email accounts set up as POP3. All my emails are accessible on each device, and stay that way unless I delete them on each device.
On iPad, iPhone and touch, I have my email accounts set up this way: Settings > whatever email account I'm using > Advanced > Remove - Never > Delete from server - Never.
I purposely set up my accounts that way because I archive emails on laptop and keep only a handful on various mobile devices. That's worked for me for years, even before iPad, because I started with an iPhone years ago.