Rather than go into detail, I'll just briefly list the ways in which this template is different from celestial.
Firstly, it can be applied to oozes. Apart from this being a strange difference, it doesn't influence LA.
Secondly, fiendish creatures resist fire rather than acid and electricity. This is probably a slight boost, though not one big enough to alter LA.
Thirdly, they smite Good rather than Evil. The uneven distribution of monsters amongst alignments means this is slightly worse than its celestial counterpart, even in an Evil campaign.
Finally, and most obviously, Fiendish creatures are Evil rather than Good. That doesn't really matter, as both alignments have useful PrC's and feats.
It should be clear that none of these differences affect LA enough for it to change. +1 LA.
Next are formians, which I'm quite looking forward to.