If you have recently updated your Android SDK to R08 (Gingerbread, 2.3) on Linux and are using Ubuntu Karmic, chances are that you will not be able to run the emulator: It will simply die with the following error:
$ ./emulator
./emulator: /lib32/libc.so.6: version `GLIBC_2.11' not found (required by ./emulator)
This is caused by an incompatibility with Karmic's installed GLIBC - the new SDK/Emulator works just fine on Lucid.
The Android team is working on a fix - in the meantime, a workaround is to download the _r07 of the tools from here, and replace emulator in the tools/ folder under the android_sdk installation folder.
A bit hacky, but works.