If (as seems likely) you're using a .conf file to change things, I'd suggest trying to run dosbox without it, just in case some change is causing problems.

There are probably other things others can suggest, but I've never needed as I haven't had too much problem with switching to a fullscreen, so I can't really say.

Personally, if you compiled dosbox from source and are still using the version you compiled in Fedora 11, I'd suggest removing it then recompiling, in case something that you compiled against in Fedora 11 has changed in Fedora 12.
If you installed dosbox via a rpm, or used yum, I don't know if an update would properly update the dosbox rpm, depending on the means of updating used. So if you didn't change things yourself, it is possible that you're still using a version of dosbox built for Fedora 11, not 12.
The best way to check would be to run 'rpm -q dosbox' on the command line and see if it says something along the line of dosbox-0.73-1.fc11.i586 as opposed to something like dosbox-0.73-2.fc12.i686
If you've still got the Fedora 11 version, I would definitely try removing it, and then installing the Fedora 12 version.

Though it is possible that this is a graphics problem. Are you having any problems switching the resolution of your computer? Can you switch to a low enough resolution?

If (as seems likely) you're using a .conf file to change things, I'd suggest trying to run dosbox without it, just in case some change is causing problems.

I've already tried that - no difference...

Personally, if you compiled dosbox from source

I didn't - I'm using the versions from the repositories.

If you installed dosbox via a rpm, or used yum, I don't know if an update would properly update the dosbox rpm, depending on the means of updating used.

It did - the version is now that from the F12 repository

The best way to check would be to run 'rpm -q dosbox'

1[vic@bombardier ~]$ rpm -qi dosbox
2Name        : dosbox                       Relocations: (not relocatable)
3Version     : 0.73                              Vendor: Fedora Project
4Release     : 2.fc12                        Build Date: Sat 25 Jul 2009 09:40:57 BST
5Install Date: Tue 15 Dec 2009 02:34:13 GMT      Build Host: x86-4.fedora.phx.redhat.com
6Group       : Applications/Emulators        Source RPM: dosbox-0.73-2.fc12.src.rpm
7Size        : 2665652                          License: GPLv2+
8Signature   : RSA/SHA1, Thu 06 Aug 2009 20:19:18 BST, Key ID 9d1cc34857bbccba
9Packager    : Fedora Project
10URL         : http://dosbox.sf.net
11Summary     : x86/DOS emulator with sound and graphics
12Description :
13
14DOSBox is a DOS-emulator using SDL for easy portability to different
15platforms. DOSBox has already been ported to several different platforms,
16such as Windows, BeOS, Linux, Mac OS X...
17DOSBox emulates a 286/386 realmode CPU, Directory FileSystem/XMS/EMS,
18a SoundBlaster card for excellent sound compatibility with older games...
19You can "re-live" the good old days with the help of DOSBox, it can run plenty
20of the old classics that don't run on your new computer!

Though it is possible that this is a graphics problem. Are you having any problems switching the resolution of your computer? Can you switch to a low enough resolution?

Yes...

Vic.

Change output=surface to output=opengl or output=overlay and fullresolution=original to fullresolution=0x0 or fullresolution=1024x768 (or whatever your desktop is using). You can add something like "echo asdf" to the autoexec section of the config file so you'll know if dosbox is using it or not (if it's in use, dosbox will print asdf at startup).