Topics in forum by link is a good way building backlinks many forum's now are offering a what they called as spamzilla where by you can spam at will.. either spam your links in the topic of the forum. i have made one in my forum. you should check it out. http://techhelp.whaitsbluebox.com when search engines bot's crawl the topic of the forum..it will be considered as a backlinks. cool!...
maybe..but it depends on how you post it.. if you post it in a spam way so it looks like its spamming..but if you post it in a professional way so it dont look like a spam..
Even if it looks good, you're going to have your link end up on a page with a ton of other links, thereby dilluting any authority or pagerank that may get passed by the initial page ... just like posting your link in a directory site.