By "digital HD TV" receiver you mean...

- DVB-S(atellite) ?
- DVB-C(able) ?
- DVB-T(errestrial) ?

Currently there is no USB device known to me that is supported by the phone itself. You would need to have a compatible driver and get that running on the N900 kernel.
Then there are the power issues. That would drain a lot more battery than you could afford...

Maybe streaming from VLC to the N900 via wireless lan would be more realistic, at least for now.