I don't know whether or not this is widespread problem, but I could find people who face this situation easily with a single search. The same applies to one of my friends; she said that she has this problem with previous and the current machine. For the first machine, she just left behind. However, she brought this to me this time.
The AC power adapter type cannot be determined. Your system will operate slower and battery will not charge.
It's what you see above; this machine was locked to run as slow as it could--always 800MHz instead of 1.73Ghz. Whether the adapter couldn't provide enough juice or the DC jack on this board is malfunction was possible here. It was really hard to decide if you didn't have a spare adapter to test. Therefore, it left me no choice but buying a new adapter to test. Luckily Dell adapter was cheap =) only $28+$9 shipping from portabledetails.com and it took 3 days to arrive.
After I got a replacement--new one has a ground line as a plus, Dell probably realized that laptop case was not the best place as a ground (electrical charge was noticeable on the chassis when I put the old adapter.)
Soon after I popped up new adapter, the Inspiron 9400 was coming alive again; battery was charging immediately and it sucks full 90W--Kill a Watt meter told me. It's really surprise that this 17" laptop draws power even more than my desktop!
Briefly, this machine is only 1 and a half years old, and surely out of warranty. If you have this problem and it's in warranty, Dell should not have any problem replacing the broken adapter for sure. In fact, this adapter basically should last longer than the laptop itself. I really have no clue how Dell took care of this design & manufacturer at first place since I never heard this problem with another brand yet. Maybe that's just me :-P
Note: 90W draw occurs only when you have almost empty battery and/or run this machine simultaneously. 40-60W is a regular power draw for normal condition.