I found a solution for this problem that hope works for more people. I run into this when updated my Samsung Captivate to ICS a couple days ago, but I think this is a general homebrew-rom Android problem, not specific to a particular phone.
Anyway, you need root access to do this and some file manager that runs with root permissions. I used 'Root Browser' to do this. The idea is to edit the apns-conf.xml file in the /etc directory and check whether there is an entry for your carrier APN. If there is, make sure the MCC and MNC values are correct. Otherwise, correct them.
If there is not an entry for it, copy one of the apn configurations that look similar to yours and change it accordingly. In order to edit the file, I run into lot of trouble with Text Editor, so I did the following: copied the apns-conf.xml file to some location in the sdcard. Plug the USB cable and edit the file with a text editor from your computer. Save it. Rename the original apns-conf.xml file to something else like apns-conf.xml.orig, just in case, move the file apns-conf.xml you just edited to /etc and check that the permissions are rw-r--r--. Change them to this otherwise.
After doing that, I rebooted and my phone auto-detected the correct APN configuration and the mobile network started working.
I know this is kind of confusing, so let me illustrate it with my particular example: I use Simyo in Spain, which is not in the apns-conf.xml file. Using 'Root Browser', I copied this file to the /sdcard directory; plugged the USB cable and edited it with 'gedit' (in ubuntu). Copied the entry
<apn carrier="Claro Internet"
at the end of the file, just before the </apns> marker, and changed it into
<apn carrier="Simyo Internet"
(Make sure to include the <apn /> markers). Saved it and ejected the phone removable disk; moved the apns-conf.xml file back to /etc using 'Root Browser' and changed the permissions to rw-r--r--. Rebooted and my APN configuration was auto-detected and the mobile network started working!
Hope this helps. Please, don't hesitate to contact me if you need further details. Sorry for the long post.