The fact that the race for the US Presidency is essentially (and unusually) between members of the US Senate (Obama, Clinton and McCain) does serve to emphasise the differences between the UK second chamber (the Lords) and the US second chamber (the Senate). One is not likely to see a fight for the premiership between members of the Lords; indeed, the possibility is even smaller now than before 1999, since present members cannot renounce their titles and hence their seats in the Lords.
However, it is possible that in the future the premiership may be held by a hereditary peer. Under the House of Lords Act 1999, hereditary peers excluded from the House of Lords may seek election to the Commons. One has already been elected to the Commons (Lib Dem Viscount Thurso). However, another effect is that heirs to hereditary peerages sitting in the Commons are no longer debarrred from sitting in the Commons once their fathers die, nor do they have to renounce their titles. There are now two hereditary peers sitting in the Commons: Michael Ancram is the Marquess of Lothian and Douglas Hogg is the Viscount Hailsham. Both are senior MPs; Michael Ancram was previously deputy leader of the Conservative Party.
It is thus not beyond the bounds of possibility that a hereditary peer may one day become Prime Minister. It would be a reversion to a much earlier period in British history, further back than the premiership of Sir Alec Douglas-Home (who renounced his peerage in order to seek election to the Commons) and the 3rd Marquess of Salisbury (who sat in the Lords as Prime Minister). The last hereditary peer to sit in the Commons as Prime Minister was Lord Palmerston, who held an Irish title.
We may thus one day again see as Prime Minister someone who is a Lord but sitting in the House of Commons.