OCLC # 100,000,000 was created today in WorldCat. In the late 70's when I first started working with WorldCat (then affectionately called the OLUC for Online Union Catalog) it had 3+ million records, so there has been a 30-fold increase in record numbers in that time, much of the growth coming in the last few years.
The main reasons for the recent growth are the addition of more cataloging from around the world, the loading of RLG records, and the assigning of OCLC numbers to article-level metadata. We've also changed the rules a bit. We no longer try to merge metadata expressed in different languages, so records from different national libraries will almost all receive new OCLC numbers, even though WorldCat might have a description of the item in another language (that is the descriptions are in different languages, even though they are describing the same item).
In the future an even larger change will be apparent. As we load institutional records from RLG, each of those records will be assigned its own OCLC number (they will also be linked to the master record that corresponds to the RLG cluster they were in). This will absorb quite a few numbers, and if extended to the rest of the cooperative, the one billionth OCLC number might well be a possibility.
Update: We pulled a copy of WorldCat shortly after OCLC #100,000,000 and found 83,286,767 bibliographic records (a 12% growth in three months).
Related post: Counting records in WorldCat
Watch WorldCat Grow: http://www.oclc.org/worldcat/grow.htm