Apart from the brake and gear problems allister listed, there shouldn't really be any problem. I've run with a 27" rear and 700c front before and it was fine. You can get low quality tyres off of ebay cheaply, but like allister said, a 700c wheel is preferable as better quality tyres are more readily available.
Apart from the brake and gear problems allister listed, there shouldn't really be any problem. I've run with a 27" rear and 700c front before and it was fine. You can get low quality tyres off of ebay cheaply, but like allister said, a 700c wheel is preferable as better quality tyres are more readily available.