I had similar problems when installing on my IBM T30 (Radeon ATI graphic card) yesterday. I installed twice with the graphical installer and I could only get it working with a vesa-driver at a low resolution. I was already a little bit disappointed. This morning I tried it again with the text installer and I could get it running with the radeon-driver (open source). It even reports the correct resolution of 1400x1050 now.
The graphical installer is very nice, but in case of any problems experienced during installation, it is still worth to try the text installer.