Everything is working fine in 3g mode the advertisement is correctly shown but as you can see from the Title when I connect my phone via wifi a got always the same response:
02-09 18:58:47.790: INFO/AdMobSDK(10151): No fill. Server replied that no ads are available (315ms)
02-09 18:58:47.790: DEBUG/AdWhirl SDK(10151): AdMob failure
The error says "no fill" which suggests that AdMob had no suitable ad to show and can't fulfil the request for an advert because it doesn't have a suitable ad. They use geolocation to choose which ads to show, and I expect that your wifi on the device doesn't provide a location (but 3G does) and so AdMob isn't able to serve a suitable advert.
I don't think you're doing anything wrong, it's a limitation of AdMob (and other ad providers) that under some circumstances their server cant provide ads. All ad-providers have a fill-rate, which measures how much of their available ad-space has been sold. It is generally not 100%, so some ad requests will not be filled.