I have found that, once Ubuntu is installed (with Ubiquity), even in the same login session in which it was installed (after clicking Continue Testing), pressing 2 or Enter no longer terminates the X server. The test system was the 20110617 daily-live on the virtual machine.

So I reset the virtual machine to its pre-installation state, cloned it to create a control, and installed Oneiric on it again while running the control, so I could examine log files from both systems side by side. Specifically, I booted both systems (selecting "Try Ubuntu without installing" from the nongraphical boot menus) and logged on with unity-2d sessions, disabled screensaver screen locking, set their screen resolutions down from 1024x768 to 800x600, installed openssh-server (and set passwords for the live session user), and then monitored /var/log/Xorg.0.log and /home/ubuntu/.xsession-errors on both systems via tail -f in nongraphical SSH sessions. I launched Ubiquity and installed Ubuntu on the first machine (the experimental), and left the other (the control) alone for comparison. After installation, I pressed the alphanumeric 2 key in each machine; as expected, the experimental machine's X server was not terminated, but the control machine's X server was.

I did not notice any differences in the .xsession-errors files that appeared noteworthy, but there were additional XKB (X keyboard extension) messages in the experimental Xorg.0.log that appeared while Ubiquity was running, and did not appear at all in the control Xorg.0.log.

This was the output of tail -f on the experimental Xorg.0.log (running Ubiquity):

[    78.985] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration profile 0
[    78.996] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration factor: 2.000
[    78.996] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration threshold: 4
[    78.997] (II) VMWARE(0): VMMOUSE DEVICE_ON
[    78.998] (II) VMWARE(0): vmmouse enabled
[    79.006] (II) config/udev: Adding input device ImPS/2 Generic Wheel Mouse (/dev/input/mouse0)
[    79.006] (II) No input driver/identifier specified (ignoring)
[    99.634] (II) VMWARE(0): vmmouse enable absolute mode
[   113.551] (II) XKB: generating xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[   200.384] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[   864.528] (II) XKB: reuse xkmfile /var/lib/xkb/server-B20D7FC79C7F597315E3E501AEF10E0D866E8E92.xkm
[   864.851] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[   878.207] (II) XKB: reuse xkmfile /var/lib/xkb/server-B20D7FC79C7F597315E3E501AEF10E0D866E8E92.xkm
[   878.267] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm

And this was the output of tail -f on the control Xorg.0.log (not running Ubiquity):

[    74.291] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration profile 0
[    74.293] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration factor: 2.000
[    74.293] (**) ImPS/2 Generic Wheel Mouse: (accel) acceleration threshold: 4
[    74.293] (II) VMWARE(0): VMMOUSE DEVICE_ON
[    74.294] (II) VMWARE(0): vmmouse enabled
[    74.301] (II) config/udev: Adding input device ImPS/2 Generic Wheel Mouse (/dev/input/mouse0)
[    74.302] (II) No input driver/identifier specified (ignoring)
[   102.897] (II) VMWARE(0): vmmouse enable absolute mode
[   116.434] (II) XKB: generating xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[   208.070] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[  2964.977] (II) Power Button: Close
[  2964.993] (II) UnloadModule: "evdev"
[  2964.995] (II) Unloading evdev
[  2965.015] (II) AT Translated Set 2 keyboard: Close
[  2965.019] (II) UnloadModule: "evdev"
[  2965.025] (II) Unloading evdev
[  2965.025] (II) VMWARE(0): VMMOUSE DEVICE_OFF/CLOSE
[  2965.344] (II) VMWARE(0): VMMOUSE DEVICE_OFF/CLOSE
[  2965.654] (II) VMWARE(0): VMMouseUnInit
[  2965.678] (II) vmwlegacy(0): Terminating Xv video-stream id:0
[  2966.313]  ddxSigGiveUp: Closing log

Most of the differences are due to the messages that appeared on the control when I pressed 2 and the X server was terminated. However, these messages appeared in the experimental Xorg.0.log without corresponding messages in the control Xorg.0.log:

[   864.528] (II) XKB: reuse xkmfile /var/lib/xkb/server-B20D7FC79C7F597315E3E501AEF10E0D866E8E92.xkm
[   864.851] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm
[   878.207] (II) XKB: reuse xkmfile /var/lib/xkb/server-B20D7FC79C7F597315E3E501AEF10E0D866E8E92.xkm
[   878.267] (II) XKB: reuse xkmfile /var/lib/xkb/server-9A8405F3FE0A780485714A4B6DD41909C2CF9F83.xkm

I don't know if this is significant or not, but since this bug seems to be in the way the X server handles keyboard input, and Ubiquity "fixes" the problem and also performs additional keymapping operations, perhaps there is a connection. Each of the xkmfiles differs from the identically named xkmfile on the other machine, so I'll attach all four, and also the Xorg.0.log and (though I doubt this is relevant) .xsession-errors files from both machines.

I don't know at what point during Ubiquity's run the 2 and Enter keys are caused not to trigger X server termination (though I'd be willing to perform additional testing to determine this, upon request). This might occur before installation completes. However, I know it does not occur just by launching Ubiquity--when I start the installer and wait for the first screen to load, and then press the alphanumeric 2 key, the X server is still terminated.