I was under the distinct impression that the Micro USB chargers and cords are being made today on a universal standard and all new build phones starting in 2011 will have that port as standard. I have some micro USB chargers that won't charge the Epic when turned off only "on". In fact if my phone is off and I plug in the charger the phone turns on by itself. Now this is not a Samsung charger but why do you suppose the phone wont charge with some of these chargers turned off?
The samsung (mostly) uses the new scheme scheduled to be universal in 2011.
1) two 5v power leads only/no data connection present: charge at less than 500 ma (in epic/galaxy s case it is less than 500 ma)
2) two 5v power leads and data pins shorted: charge at above 500 ma (in galaxy's case about 700 ma)
In addition we are seeing the following which are not part of the standard but are sammys proprietary to to allow more options:
3) two 5v power leads and data connection preset: charge at less than 500 ma and trigger of usb connection menu (mass storage, tether, etc)
4) some unknown configuration for usb media dock (turns phone on among other things)
5) some unknown configuration to be used for car dock
6) some unkown unfinished configuration for usb to hdmi.
my guess is op's non sammy charger, where he is seeing forced on, has some level of resistance or some other pinout shorting that is signaling epic for #4 through 6.
FYI there is a seventh condition/indication, a message that "battery is not connected", which doesn't have to do with the charge but indicates a faulty battery.
my own suggestion if you have no ability/interest to test pinouts or rewire, is that you get a charger made for a recent phone. eg a galaxy, or a palm pre, or a epic, and you will have what you need.
If you do have an old charger around that is not shorted properly and only charges <500 ma, don;' toss it, that is good to use next to your bed where you probably only need a slow charge.
Most people woudl definitely want a good >500 ma for their car where nav demands (continuous screen, gps, 3/4g, processor) may result in slow/no/loss of charge at 380 ma.
the good news with the standards is investments in good proven chargers will last at least this phone and your next (universal induction will be common in five years) as well as be usable for the phones of friends.