Problem #2 came quite a bit faster. The yield syntax in F# is very similar to the C# iterator syntax and made translating this sample a breeze. 

As a commenter posted, in problem #1, I would've been better served to use Seq.filter as opposed to Seq.choose for the purpose of filtering list.  I've applied that here.  One downfall of learning a new language is finding the right API.  I spent a bit of time reading through the Seq documentation looking for the equivalent of the Where extension method and could only find chose.  Lesson learned

// Find the sum of all the even-valued terms in the sequence which do not exceed four million.
let problem2() =
    let rec fib prev cur = seq {
            yield prev
            if cur < 4000000 then
                yield! fib cur (prev+cur)
        }
    fib 1 2
        |> Seq.filter (fun x -> 0 = x % 2)
        |> Seq.sum