Without mentioning the N900, ask O2 whether this unlimited-data plan applies to the iPhone. A lot of providers exclude iPhone usage from their standard unlimited-data offers. They tend to be the same providers who exclude tethered access. (You say that O2 makes that exclusion.) It is a reasonable bet that the same providers will exclude the N900, and will have a special (and more expensive) data plan for it, as they do for the iPhone.

Unless O2 are already advertising the N900, there's no point in asking about it. They will not have a policy in place yet, and the question will just confuse them.

Or you could wait, and ask your questions about the N900 when answers are likely to be available.