Re: Irimi or Omote?!!!
I think the distinction given by Jun is correct; it seems to me that 'irimi' and 'tenkan' translate most usefully as 'entering' and 'turning', while it's most practical to think of 'omote' and 'ura' as indicating relative position (in front, behind).
Some of the confusion arises from the fact that *mostly*, when you execute a 'tenkan' technique you happen also to end up 'behind' uke... so it can be tempting to infer that they are synonyms. (Incidentally, when I first started, I thought for ages that "omote" meant "get on with it"... purely because it was usually the last word uttered by sensei before he expected us to start practising...)
I think a good illustration of the distinction is this: think of doing shomen uchi ikkyo (irimi): you 'enter' towards uke, and pass to his/her front, not his/her rear.
However, now think of shomen-uchi irimi nage: your first movement is still 'irimi', but it will bring you to a point behind uke, not in front. So 'irimi' can be 'omote' or 'ura'...
By the same token, you can make a 'turning' movement (tenkan) in front of uke as well as behind... it's just that standing in front of uke and turning your back to him/her does not usually recommend itself. (One case where you might do this is in simple evasion exercises against a shomen strike with a bokken...).
Hope this helps....
By the way, confusion over these terms is not confined to the Japanese-English direction of translation:
About a year ago, I heard a visiting shihan explaining the 'ura' version of a technique through his interpreter. Deciding to exercise his English, he referred briefly to the 'backside' technique... until she translated that back into Japanese for him. There was respectful mirth.