I'd be inclined to suspect a possibly faulty phone, or a suspect battery. That's assuming the problem isn't entirely visual (aka the battery is definately not at a low state) - since there are still lots of weirdness on IOS 4.0 giving strange indications (try googling and see what i mean) and strange indications are amongst them. Note, that's not related to the warmth bit. But if it's not an IOS issue :-
It's worth noting that most types of accumulator cells (aka the cell type used in rechargable battery packs, which whilst differing in type, are all accumulator type cells in general) tend to get warm when they are put into a full charge or a forced fast charge - depending on the number of cells in the pack (to use one factor as an example) the combined pack is prone to getting warm, so that in itself is not unusual - and remember that the metal casing of the iPhone (and Touch, and many other devices of similar case construction) is a good heat conductor, so the leaking heat from the cells (which are in a non-insulated pack in thermal terms - the pack is electrically insulated though) will tend to dissapate through the case.
Although i say warm is fairly normal in general, for a short period after the charger has been disconnected, it should never be so hot as to feel like it could burn.
But the indicator of a potential fault would lie with the fact that the indicated charge falls to a very low 25% remaining indication. This can happen on some devices when all their transceivers are active (in the case of the iPhone, you have transceivers for WLAN, BT & Cellular radio), but a sharp drop from a full charge inside of an hour (idle i mean, not in use) like that isn't a good sign.
I'll assume you let the phone have a full charge from the first time you charged it, and let it fully charge for most of the times it got charged in that 6 days. The reason i say that, is the lifespan of an accumulator pack is highly defined by how you treat them - some types (Li-Ion & Li-Poly - Li-Poly being the case here) usually settle to their full characteristics after the first few long charges - where older types (Ni-Cad in particular) tend to take a good few charges to settle to a stable characteristic.
So assuming you gave the battery a fighting change to charge & settle over the first few charges (never a bad thing with accumulators, often pays dividends later in the packs's life), and whilst sitting on the table it's not powered up and under a heavy load - then there is potentially something suspect (hardware wise, device or battery or both).
The only other notable thing i can suggest, is if the phone is sitting on it's back (aka laid down), the antenna is directly facing a solid object (which at the frequencies in question, is comparable to a lead-lined container relative to radioactive material, a brilliant attentuator) which actually forces the radio transceivers (especially the Wlan & Cellular ones) to poll more frequently to find networks etc or to reestablish connections due the fact the signal is relatively poor compared to if the phone was sitting upright (where there is no directly obstacle between the antenna and free air). The upshot of that scenario, apart from the radio transceivers getting warmer quickly (due to a higer duty cycle of operation) is a distinct increase in current being drawn from the battery pack.
So the red indication could be indicating a heat issue or a too-high current demand from the battery back (a lot of Li-Ion & Li-Poly cell packs do monitor usage etc, so it's possible to determine automatically if the discharge current is too high) - but that you'll have to check in the manual (it should mention some caution under trouble shooting for red indicators if they are significant).
If in doubt, given it's a week old (or so, by the time you get around to looking into the matter or other suggestions), it might be better to take it back as suspected faulty, unless by chance one of the scenarios outlined is the case - but i'd still be suspect about it all, based on what i know of electronics and radio transceivers and battery types.
Good luck, and shame if it proves to be a faulty item.