If you haven't yet returned your Inspiron, I'd reconsider your decision.

I've had an Inspiron 8200 for about 13 months now, and love it. It too consumes 90 watts of power, but there is a Lind Auto-Air adapter for it (model DE2035A-259A). You can order it directly from Dell. It says "Designed for use with Dell(tm) Lat. C Family & Insp. 2500, 4000, 8000)". Since it was made over a year ago prior to the 5000 series coming out, that's why I suspect that the 5000 is not listed on the list of supported devices. But, it would not surprise me at all if it works with the 5000 or in the worst case that they have a separate model that does.

By the way, my 8200 also has a 90-watt power supply. The Lind has a maximum output of 70W, and the Inspiron is smart enough that it detects that this is the case and limits itself to 70W current draw when operating on the Lind. In fact, as soon as you plug in the Lind power supply, a BIOS message pops up on the screen explaining exactly what I said above. I'm not sure exactly what it limits itself to, but it may be as simple as not charging the battery while at the same time running the computer at full screen brightness, running the CPU at full speed, and burning a CD-R all at the same time. I've never noticed anything different when it's running on the Lind except for the initial warning message when you plug it in.

As far as what you were told by Dell, it's entirely possible that the person you spoke with was not familiar with the details. And as far as letting you return it, Dell has a Total Satisfaction Guarantee on all of their products for 30 days, which allows you to return computers for any reason (or for no reason) within that time period, so the notion that you "caught them" and they're letting you return the unit doesn't hold water.

Also, do you think that you're the only one that needs to run a Dell laptop on an airplane? They have many thousands of customers that do the same thing every day without problem. It's unfortunate that you didn't get a hold of someone there that was familiar with the proper solution to your situation.