Could we see R600 by the end of the year? It's a possibility, as ATI has finished producing samples of the cards for development and now the manufacturers will be able to start producing the cards. We may see them by the end of the year, which is fairly soon considering how short ago it was that R600 was first demoed. Many people are looking forward to it, especially because of how good it looks on paper:

According to sources familiar with the plans of ATI Technologies as well as some media reports, ATI R600 will have 64 unified shader processors - an unprecedented number so far, 16 texture units - inline with today's GPUs, clock-speed beyond 650MHz and support for high-speed GDDR4 memory controller.
ATI isn't exactly known for their on-time releases, though, so don't expect miracles here. However, seeing new cards out before the holidays would be great, especially because of how it would impact the pricing of current-gen cards (which is still astronomical at the enthusiast level).