It happens a lot up here in Yorkshire, including the big names, ie;
Alistair & Jonny Brownlee; both members of 'Bingley Harriers', & started their (out of School) competition life Fell-Running at the Woodentops events at Haworth (& still do - Jonny was there on New Years Eve)
http://www.photos-dsb.co.uk/ALS 2012/content/FV4Q0815_large.html
http://www.bbc.co.uk/news/uk-england-leeds-15175693
Rob Jebb; who can argue with his pedigree in 'cross, 'fell (for 'Bingley Harriers' & and as a GB team member) & 'XC'?
Richard (& Robert) Thackray; the ex-GB squad cross riders, who rode at the World Champs/ex-Pace Research sponsored. They both run fells as well as 'cross/MTB
Rob's also involved in Triathlons
Chris Young; ex British Cross Champion, used to run fells
Tim Gould; ran some when he was in his prime (6 times '3 Peaks Cyclo-Cross' winner)
Please don't get me wrong, I still ride - even if i no longer race.