adding the dpi to the drawable is invalid. You should just be able to set it like so in the manifest:
<Application
...
android:icon="@drawable/ic_launcher" >
<!-- change ic_launcher to whatever your png is named -->
The typical folder structure is as follows:
/
AndroidManifest.xml
/res
/drawable-ldpi
ic_launcher.png # 36x36
/drawable-mdpi
ic_launcher.png # 48x48
/drawable-hdpi
ic_launcher.png # 72x72
Once you've done this, you'll probably need to do a clean (eclipse -> project -> clean), but you'll know it was successful if a line like this appears in your /gen/R.java
file:
public static final int ic_launcher = 0x...;
If it isn't, you can delete the R.java file and the ADT will immediately panic and rebuild it from scratch, incorporating your new drawables.
As a final note, sometimes the emulator seems to cling to the old icon - the clean should make it change, but if it doesn't, go into settings and uninstall the application and then run again from eclipse, everything should REALLY be clean (the same applies to actual devices as well)
NOTE the answer now includes the comments below