Etched Forever: First, and most important, you are stuck with the engraving, so if you absolutely must engrave the iPad, double check not only the spelling, but also that the message is OK with the recipient. If you are buying your son an iPad, does he really want Jason's iPad written on the back, like you used to do on his lunches and underwear?
Returns: If you purchase an Engraved iPad it's yours, no take backs, no returns, no exchanges. The iPad engraving policy doesn't lay out the terms as clearly as the "Personalized iPods" return policy, but we expect the terms to be similar. The tighter return policy is actually a motivation for Apple and others to offer free engraving. Without the ability to return items that aren't damaged, the companies save big; in 2008, Sony announced that free engraving had saved the company more than a million dollars!
Reselling: If you personalize your iPad or iPod, expect to take a hit when you go to resell it down the line. Think about it. Would you want an iPad that said "To my one and only. Love, Doug" on it? Even if you plan to recycle the machine down the line, you'll take a hit. With the current iPod Touch, personalization will cost you $10 when you resell it to Gazelle.com.
It will be in a Case: The final reason not to etch, and jeopardize resale value and returns, is that the iPad will likely be in a case for most of it's life, covering up the special saying until you want to resell it. Whether you're getting a case to protect the iPad, stand it up or add some color, the case will cover the engraving.
If you want to personalize an iPad, check out a company like Gelaskins, which makes a laminate vinyl decal that is backed with 3M adhesive. You can choose from a wide array of artist contributions or upload your own photo for personalized protection that will protect your resell value, instead of harm it. Gelaskins for the iPad start at $19.95.