There is nothing wrong with having a < before or a > after it.. so long as the < has a > after it before and vice versa for after.... so I don't think yours will quite work...
I came up with the following... which seems to work at least somewhat...

s|(\Q$text\E)(?![^<]*>)|<I>$1</I>|gi;
[download]

which matches the text so long as it is not followed by a > before it sees a <

Update \me laughs as joealba and I both post the code I gave him at the same time...

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