Before we get into the implementation, let me start by saying that this is more of a high-level marketing decision than a technical one. The important thing is from a technical side – it can be done. This post should help determining which option is right for you and your organization. You may need to involve a marketing person in helping you obtain the answers needed. If you need mental preparation to deal with your marketing team, go through a Dilbert calendar.
Do your global sites have their own identity? If so, multiple domains may be the way to go – especially as far as search engine optimization (SEO) is concerned. If there are search engine marketing (SEM) campaigns being strategized and/or individual content campaigns being designed for each site, the campaigns may end up easier to manage if you have multiple domains (e.g. mysite.com, mysite.co.uk, mysite.com.fr).
You can also use subdomains, which may be less costly from an SSL certificate perspective, discussed below, but there may be cultural considerations. If you are going to have a large audience in UK, for example, where the dominant domain suffix is .co.uk, how many will know to access their localized site using uk.mysite.com versus mysite.co.uk? How many subdomains can you name at the top of your head?
If the multiple-domain setup is the chosen route and you are planning to collect user data such as registration information, credit card, or any other sensitive information that may need to be served via the https protocol, you will need to decide what kind of certificate you will need. If you are doing subdomains (e.g. france.mysite.com, spain.mysite.com, etc.), you may be able to purchase a wildcard certificate that will work for all sites ending in *.mysite.com. This may end up less costly in the end than purchasing individual SSL certificates for each domain, but there are the cultural questions to consider from above.
From an IIS perspective, if a subdomain is your ticket, you are able to host the sites from a single IIS instance. For separate domains, you will need to set up separate IIS sites pointed to the same location. In addition, if you are hosting the sites from the same IP, you will need to set up SSL bindings using IIS Admin Tools. The detailed step-by-step commands are outlined in this MSDN article: http://bit.ly/bOtGXB
Multiple Domain Implementation in Sitecore
The good news is Sitecore makes it real easy to implement the multiple-domain scenario. All that is required are some minor tweaks to the web.config and some optional code to implement depending on how you form your links. I will go into detail in the a post shortly following this one.
On the other hand, if all you are doing is providing a translated version of your site in multiple languages with minor content differences, the single domain implementation may be better for you, also making the SSL problem easier to solve with just a single certificate. In addition, no web.config tweaking is necessary. You are ready to go out of the box.