There aren't many companies you guys have heard of available to invest in. Sportingbet is one, SBT on the London market. Cryptologic, the software providers, are traded on NASDAQ with symbol of CRYP. A bunch of major British bookies are traded in London, but they don't operate books for Americans, companies like Stanley Leisure and William Hill.
Nigel Payne, the CEO of Sportingbet recently claimed he has heard of 15 companies planning to go IPO this year in the field, all because of the obvious high valuations. PartyGaming, the parent of Party Poker, has said they won't go public this year because they don't need the money and obviously want to generate an even higher valuation. Don't be surprised if they are patient and things continue that they reach a $7 billion value when they go public.
The UK looks very likely to pass the online regulations which will allow for US bettors. While all books and poker sites may not decide to go to the UK because of relatively high tax rates, the UK will clearly set the bar for regulation and just about every jurisdiction will follow allong and allow US bets in short order once it starts running. The US government won't be able to do a damn thing about online gambling in the very near future unless they really want to betray and piss off one of our best allies over something as trivial as online gambling.