Is this cloaking

[Langers]

I have about 30-40k of ads on my site and I found that lots of my bandwidth was being used by robots crawling my site. As a result I have replaced the ads with blank images of the same dimensions for the search engines to see.

I have also replaced a javascript dropdown menu with a list of links for the robots to see (and follow).

Does either of these count as undesirable cloaking techniques to the SEs?

[MrSpeed]

Technically yes. If you are showing different content based on IP or user agent you are cloaking.

In your case you have a "custom content delivery system based on specific users needs".

Why cloak at all?
Spiders do not request images so showing blanks to them will not reduce your bandwidth. Also in this day and age bandwidth cost should not be an issue with just about any site. Even minimal ad revenue is enough to cover hosting costs.

There are also work arounds for the dropdown navigation such as sitemaps, duplicating the links at the bottom of the page etc..

[volatilegx]

While your techniques do fall under the definition of cloaking, I disagree with mrspeed about whether they would be considered undesirable by the search engines.

I think even if your pages were reported as spam by a competitor, a human review would conclude that they are OK.