In the Internet Marketing Niche, you have a definite advantage because everyone is looking to make money, and you have great places like the Warrior Forum to make great contacts. It might be a bit harder in other niches, but it's still definitely possible! Contacting them for JVs directly is probably the best option, but have an offer that'll make it worth their while. Also, try to keep in mind that you're building a relationship with them so you can possibly do many more JVs with them in the future, so make sure to give their customers a lot of value.

As for the other question, why not have both? It's a definite advantage to having a content website so your customers can keep coming back for more (make sure your opt-in form is on every page), but having just an opt-in page might be good for giving away free content to gather more subscribers from the search engines, give aways, JVs, etc.

I guess people look like they are mainly doing ad swaps in IM because its a widely covered subject compared to the **** berry niche. However it shouldn't mean you cant be successful in this niche for ad swaps.

I personaly would create an email with good grammer and email to the site owner as you suggested. Explain what you would like to do and tell him the benefit for his/hers site and they would be getting exposure out of it too.

regarding your opt-in page question: I do have websites where the home page is just an opt-in but for your berry niche I would recommend you have an optin sub page and also the optin in the side bar which will appear on every page. Then create plenty of unique content that people feel like are getting good info from, they will then be enticed by your opt-in on the sidebar for which you might offer a report or ebook.

Hope that helps