Are you sure you're running 64 bit Ubuntu? Try running the 32 bit version, and please paste the output of 'uname -m', without the quotes that is!
And seeing that you're using 8.04, which is quite an old version as 10.04 is the current stable, I suggest you update your system.
Please note that Debian Lenny users also have the same problem, I guess this will be fixed in the next release.
This is not a bug, it's an error of the programmer who compiled this.
Bitcoin was built on Ubuntu 10.04, which is more bleeding edge than, well... 8.04 and Debian Lenny!
I guess this will be resolved in the next release.