I don't think you assigned a Linux Swap partition.
The way I installed Macbuntu (Ubuntu) 11.04 was like this.
1: Carbon Copy Cloner Lion to a external HFS+ drive, option boot from it, Disk Utility erase and set up the partitions.
A: OS X Lion- HFS
B: OS X - Snow Leopard - HFS
C: Linux - MSDOS/FAT32 (changes to EXT when installed)
D: Linux Swap - same as C (very small)
E: Shared - exFAT
F: Windows - MSDOS (change to NTFS in Windows installer)
Reversed cloned Lion into A, booted into it and installed rEFIt, reboot twice for it take hold
Reversed Snow Leopard from a clone
Installed Ubuntu 10.04 from cd, assigned the Swap partition and formated, once inside, then upgraded to 11.04 which defaults to the classic desktop, installed the drivers and the fan control, (then for Unity 2D requires downloading it the Ubuntu Software Center and selecting it at log in.)
I suggest you do the 10.04 method outlined here, then upgrade to 11.04 in Ubuntu.
Direct installing Linux on a Mac is even harder than on Windows, there are less people doing it, so there is less help, less driver support and Ubuntu isn't designed to install on a Mac, just a blank PC box.
To finesse something else requires a lot more in depth knowledge. Good luck getting much of any help on thier forums.
In my opinion if your just curious and just staring out with Linux, is to install either the free VirtualBox, the paid VMFusion (best) or Parallels (very good) virtual machine software.
What this does is allows you to run Linux (or Windows or even another Lion in Lion with VMFusion) in a window on OS X.
ll you need to do now is simply download the right ISO and point the VM software at it and let it rip.
This lets you check other operating systems out, BEFORE doing some potential bricking of your main machine, leaving you stranded with no way to get online.
Really all you need is Apple's Bootcamp for a Windows partition to run 3D games in there as it needs the full hardware.
Linux can stay in a VM in OS X for ease of access.