How to legally bind users to T&C without registration?

Allowing users to access and utilize, even if in limited fashion, your flash gaming website can serve as consideration to support their obligations under contract (i.e., the Terms of Service, or ToS). A user's "registration," what ever that term could mean, would not necessarily be a prerequisite to you being able to enforce your ToS against him.

The larger concern will involve the content of services you are offering over your website (e.g., gaming, as in gambling?) and how you may (i) best limit your liability to users and (ii) ensure you are not running afoul of various state statutory requirements, like Cal. Bus. Prof. Code Section 22575.

Terms of Service and Privacy Policies are certainly not one-size-fits-all instruments; consulting an attorney to review the contracts you are proposing to execute with the anonymous internet population is advisable.

Listen to yourself, if in the realworld you walked into the door of an unmarked gambling hall, are you suddenly bound by anything which is enforceable by law? The basic truism of a contract is that there must be a meeting of minds. Now if you were part of a web-ring and you were doing convention gaming activities and the user was a regular participant on other sites then you can ask on grounds of expected and habitual trade/business practices but if the user was an underage newbie who'd lost their shirt, the courts tend to take the side of the less powerful in any dispute. The classic contract of adhesion case was a sign BEHIND a hotel door with some rather stringent clauses which the judge held not to be a contract.

Is it really that hard to point out via random popup that people can play anonymously but get extra benefits with registration (along with click-thru acceptance) and protect your own back? Paint it as positive, not a drag.