With a million units reportedly shipped in its first weekend, there’s no doubting the iPhone 3G S is a hot gadget. Unfortunately it turns out some units are slightly too hot.

Several sources, on both sides of the Atlantic, are reporting the device has overheated so badly it actually changes color. French site Le Journal De Geek, published a reader image (pictured above) showing a phone with a distinctive marking which appears to match the outline of the battery.

Meanwhile Melissa J Perenson of PC World notes her phone not only became hot at the back (where the French reader reported problems), but the heat spread to the front and reached the point where it became uncomfortable to hold to her face.

Wired notes that some reports pinpoint the problem to being worst while using GPS capabilities or playing games. That leaves the cause open to speculation as these could be down to either a specific component or the main processor.

Apple has yet to issue any public statement on the issue, but has asked the French user to return his handset for inspection.

Ironically the overheating reports come despite the withdrawal of the Hottest Girls application. The app, the first to take advantage of parental control settings by including pictures of nude women, is no longer available. Officially Apple pulled the plug, saying the developers had changed the content after approval without permission. However, the developer beat them to the punch by removing the app first, citing excessive server demand.