Usually, the longer you wait after an Apple product introduction, the shorter the ship time becomes. That's not the case with the new 21.5-inch iMac. When it first went on sale shipping times for all but the first-day buyers were quoted as seven to 10 days. Shipping times remained at seven to 10 days through the busy holiday season as well. Today, shipping times for the new 21.5-inch iMac have increased to between two and three weeks.
This increased shipping time first appeared in Apple's Asian online stores before migrating to Europe. From my vantage point in the UK the Apple online store is also now quoting two to three weeks. As of this writing the increased ship times have not propagated to the US store, but purchasers there can expect it to happen shortly if the past is any indication.
So what does the increased shipping time mean? It could be that demand is very high. It could also mean that Apple is having a problem obtaining all the parts they need to keep up with demand. It could be a bit of both. The all-new, super-slim iMacs have been in both high demand, and have had part supply issues since their unveiling in October.