A theme based link from a high PR site back to your site would also have the same effect as that from a .gov or .edu site. I completely agree to the fact that .edu and .gov helps but when it comes to acquiring a link back from this site then it becomes very difficult to do so. So you may consider linking back from a high PR theme based site.
Any high PR links helps. .edu and .gov links are usually high PR quality sites... not because of the domains that they are on but the trend that they follow (and thus have people asking for .GOV & EDU domain sites since they think it's the other way around ). So just look for any high PR sites to link from period... ideally one that relates to whatever your site's theme is on.
It would be best if you used websites wit a high PR that was also related to your website, if Google see's that the website is not related then i heard that it may not count towards being a valid link and therefore wont contribute to your PR. I am not sure on that, so dont take my word for it. Will.
Don't even worry about them. .edu and .gov TLDs have no more inherent weight than any other top level domain. Just focus on creating and maintaining a great Web site with killer content that other sites would kill their designers (and their designers' mothers) to link to. As for backlinks, don't worry about getting lots of backlinks in bulk. 9.9975 times out of 10, those links will be worthless to you anyway (bad neighborhoods, low PR, not related to your site's market, niche, or topic, etc). If you haven't done so, I strongly suggest you read the free Search Engine Optimization FAQ over at SitePoint. Paying a visit to the SEO thread linked in my signature would also not be a bad idea.
The former will help drive targeted traffic to your site (which is never a bad thing), but won't do much for your rankings, at least not directly. What it can do is bring your site to the attention of those who would consider linking to it, which would be an indirect benefit; plus those links will be worth far more than forum links by themselves anyway. As for content, the better, more relevant and targeted, the better. Good luck with your site.
Quoted for truth, many people still didn't recognize it, but after a few really ugly months of .gov and .edu spam ranking high all over the place, google apparently pulled the plug for this tactic...a great move !