According to a report from Peter Kafka and Kara Swisher at All Things D, Yahoo CEO Marissa Mayer has her sights on wildly popular blogging platform Tumblr.

Quoting “sources close to the situation,” Yahoo may be looking to invest in, partner with, or even outright buy Tumblr. According to sources, talks have been “serious,” but as you know serious talks does not a deal make. Mayer, who has been watching Tumblr since her days at Google, reportedly started up talks with Tumblr execs, including CEO David Karp, soon after taking the CEO job at Yahoo.

One thing is for certain about a Tumblr acquisition – it wouldn’t be cheap. As Tumblr continues to grow (they just hit 100 million blogs a little over a month ago), the platform will only attract more attention. Estimates value Tumblr at $800 million at least, with some suggesting that it could be a $1 billion company.

On the surface, it’s clear to see how a Yahoo and Tumblr deal could benefit both parties. Since Mayer took over Yahoo, the company has been going through a shift – mainly in an attempt to regain some relevance that it has lost over the past few years. Tumblr is one of the hottest things on the internet, so that can’t hurt. Plus, Tumblr would benefit from even more exposure. If there’s one thing that Yahoo does have a ton of clout left in it’s the Yahoo homepage. Yesterday, Mayer announced a partnership with Twitter to bring real-time tweets to the Yahoo newsfeed. Tumblr could benefit from this sort of increased promotion.

Since Mayer became CEO, Yahoo has been busy making small acquisitions – Summly, Astrid, Milewise and GoPollGo. But a Tumblr buy would be a huge deal. We’ll be anxious to see how this one develops.

UPDATE: According to GigaOm, Facebook may be circling, looking to snatch Tumblr out of Yahoo’s hands.