Upgraded to Lion this morning which went mostly smoothly, but when I tried to boot back into Ubuntu (I do most of my work in Linux) I was only able to get as far as the grub command mode.
I spent a good portion of my morning following through guides for similar problems on Windows machines, booting from an Ubuntu install disk and re-installing grub, but I am still unable to boot into my Linux partition.
During my upgrade, I noticed the installer tried to repartition my disk to make space for something.
Maybe it has overwritten your grub stage 1.5, so now you cannot load configuration and drivers; or maybe it changed the relative position of your Linux partition. In these cases, you will probably see grub rescue mode which takes some guess work to resolve. If it's in normal mode, you can use the ls command to find the Linux partition and edit your boot entry accordingly.
An easier way is to use the LiveCD to boot and re-install grub. However, I am not sure if it can install the EFI version of grub for you. You might have to use the MBR version in BootCamp instead.
I ended up with a solution that was really simple and easy, but resulted in a little bit of data loss.
I had given up on grub after many failed attempts at re-installing from the livecd and was resigned to just reformat the partition and re-install ubuntu, and noticed the option to re-install 11.04 on top of my current copy after starting the installer.
I re-installed, and must have gotten a new copy of grub set up correctly during this step. Only side effect was most of my applications I had installed through the softwarecenter were missing, but when I re-installed them settings were still there.
After 20 or 30 minutes of re-installing applications rEFIt and both OS installs are working the same way they were before the Lion install.