For most people the 16g is fine as a first iPad. Computing is increasingly living in 'The Cloud' - ie Google Docs- and the iPad is designed on this assumption. Serious photographers who want to monitor their photos in the field, should get a 64g iPad: otherwise, save your money. You will inevitably need more space eventually, but by then there will be the next generation of iPad to upgrade to.
Think of your device as a communicator. With wifi, all that you own is accessible, your photos on Flckr, your music on iTunes (£20+ per year) your videos on Vimeo, your documents on Dropbox. Without wifi there is a problem accessing, so really important stuff would be best kept locally (ie your iPad). If you have a smartphone you can tether your iPad to it when you can't access wifi or you can check out 3's mifi.
Some Apps are becoming more sophisticated- ie getting bigger- and this may become an issue in the future. However, you can delete them from, and restore them to, your iPad whenever you wish. Most apps do not take up masses of space. Some graphic intensive games come in at over 1gb, as do the new interactive video rich books that are just being introduced to the book store, as do the HD movies downloaded from iTunes. A single RAW photo file imported from your SLR might be 20MB. If you are planning on this sort of usage then 16Gb will fill up quickly. However, if you want to transfer the photos from your phone, download a load of books from the Kindle Store, or install a selection of the apps mentioned in this wiki, then you will find that 16gb will go a long way, especially if you avoid clutter.

A note about storage and backup:
When you set upyour iPad you are given 5gb of free backup space with the option of buying more. You can also choose to backup to your iTunes on your home computer. The advantage of the former is that you know that all your data is always automatically backed up; the disadvantage is that you might run out of space and not want to opt to pay for increased storage. The answer then is either to delete ( you can always re-install from the app store) apps you arent using or to manage your apps through Settings.