With the current 1.0.25 ALSA release you need to do some hacking to get Scarlett working. But the hack is as simple as it can be.

Download the tarball ftp://ftp.alsa-project.org/pub/driver/alsa-driver-1.0.25.tar.bz2 and comment the call to snd_usb_create_mixer() on the line 517 in alsa-kernel/usb/card.c so it would look like:

if (err > 0) {
    /* create normal USB audio interfaces */
    if (snd_usb_create_streams(chip, ifnum) < 0 /*||
        snd_usb_create_mixer(chip, ifnum, ignore_ctl_error) < 0*/) {
        goto __error;
    }
}

Build and install:

$ ./configue
$ make
$ sudo make install

Considering a bad practice to use sudo make install, in our case you don't need to worry. Just reboot and select Scarlett as a default device in Sound Settings. Voila!

To restore Scarlett after reconnecting it from the USB port use:

$ sudo alsa force-reload
$ alsactl restore 0