Judging from the responses in this thread, people have had success using Nottingham up to 10% abv. I think I heard 12% somewhere else, so maybe it can go a little higher. If you're doing a high gravity beer, make sure you're pitching at least two packets, though.
ya I did a big pack of the yeast and I'm sitting at about 10.5% according to my OG and FG I was concerned that I wouldnt get any carbonation in the bottles with it being so high.
With that big of a beer you will probably want to let it condition for quite sometime before you bottle. It is common in big barleywines to have to add additional yeast to naturally carbonate in bottles.