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I usually endup tweaking a new page several times over the course of the first week or 2 that it's up. So if I waited to post my final revision, it might take a while. If you put up a coming soon page with a decent, unique title & maybe a "This page about bluse fuzzy widget repair centers in Smyrna coming soon" you may get some nice ranks/visitors earlier.
rmjvol
Personally, I will spend a few hours writing up a nice page of content explaining what is "coming soon." Really, it only takes a few hours if I still need to build an overall plan. Once I have a good plan, it only takes 10 - 15 minutes and is well worth the time.
Welcome pages?
If that means the same as a splash page, then no, I don't do it. I want to give the visitor exactly what they are looking for or expecting ASAP. There are exceptions to that, but I think they are few and far between.
Use lots of H1 tags, bold, etc. to make sure those pesky SE can read the page and index it properly. This coming soon page should contain no HTML developed later than 1996 and be a very ugly text only page.
Then when your site is good to go, swap the "coming soon" index page and with the final version index page with all the bells and whistles and bingo you have good rankings and traffic right off the bat.