It is not a problem, just the way they were made. I have an Inspiron 8200 with a 1.9ghz and remember that detail. It will work with the older lower wattage (70W) ac adapters but only at the slower speed and you need the higher wattage (90W) ac adapter to get the higher speed. It had something to do with the new (at the time) release of the 1.6ghz and up P4-M processors.
The other issue that arose with those laptops was that the on board connector for the AC adapter had a bad solder job. Mine had this problem - thought it was a bad AC power supply so I bought a second one. After time, it gets a little loose and the AC power supply doesn't always work. Sometimes you can wiggle the connector and get it to work, but it's only a temporary fix. I actually had completely disassembled mine to get to the board and I found the connector that was loose. Just added a little solder to it and it works great ever since. It will power just fine through a docking station if you have this problem, as the docking station powers it through a different connector (used this solution for years until I found out that the problem was a bad solder job at the factory).
If you want, I can dig out my laptop in the morning and get the part # off of the AC adapter.
Edit: You can buy a brand new one here http://www.kahlon.com/itemdetailpg.asp?itemid=912790