I saw recently that the i386 CPU is not going to be supported in the Linux Kernel 3.8 so does that mean that Ubuntu will only be available in the coming years in 64-bit?
No, it means that you won't be able to run Linux on the Intel 80386 chip, the original chip from Intel which supported 32-bit architecture. Those processors had clock speeds of 12-40 Mhz and were superseded by Intel 80486 and then Intel Pentium in early- and mid-1990th.
Other, more modern 32-bit chips (Pentium Pro and above) are still going to be supported. Those processors are characterized as having the i386 architecture but they have some improvements over the original 80386, the use of which allows the removal of some ugly crutches from Linux that are required for 80386 support:
Anecdote has it that there was some sentimental value in supporting 80386 processor as it was the processor that Linus Torvalds used when he developed the first version of Linux.
Also, despite i386 support present in Linux kernel up until version 3.8, in the recent years most Linux distributions were compiled with settings which made them incompatible with 80386 processor anyway. In particular, Ubuntu requires i686 (Pentium Pro) processor or above since Ubuntu 10.10.
Ubuntu hasn't run on proper "i386"'s for a long time, support for the processor was dropped in 10.10.