There's lots of good accommodation in the area, from camping and bunk houses to posh hotels. If you have any recommendations, please add a comment below.
To be really close to the action you can camp in Cwm Llwch in the Pen y Fan race registration field by kind permission of the Powell family at Modrydd Farm (01874 622891). They will tap you for a reasonable sum in the morning. NB no facilities - please look after this beautiful valley and take your litter home!
The Powells also run the Penstar bunkhouse and cottage, situated within a mile of the Pen y Fan race registration field, making a great base for families, groups or clubs coming to the races. See http://penstarbunkhouseandcottage.co.uk
These links may be helpful:
And this map shows the race routes and some local amenities: