Live plants in an aquarium have many benefits: they are nice to look at; they can be centerpieces unto themselves; they provide natural places for fish to seek protection and to spawn; and healthy plants help maintain a healthy water quality. This article will focus on one "secret" additional benefit of live plants: their use during the initial cycling of an aquarium.
A few caveats before the discussion: this article is not about how to grow plants, and I do assume that you are starting with healthy plants with sufficient light and proper substrate. Also the fish in my tanks are not any types known to be "plant eaters" (which, in my definition, would include any sucker mouth catfish besides an Otocinculus).
That being said, this article is about my experience in cycling aquarium tanks in substantially less time than with "traditional" methods. The four tanks referenced here are two 10 gallon, a 20(long) and one 38 gallon. In each of these cases, I was able to moderately stock the tanks in about a week, and add the remaining community fish at the end of the first week without measurable second spikes of ammonia (NH4), and no measurable Nitrite (NO2) ever. The secret? No real secret. I use a double-barreled approach: live plants and seeded gravel.
Seeded gravel is, simply, gravel from an established aquarium. In all four tanks I used a minimum of one cup of gravel (in the 38, I used 2-4 cups). What do you do in the established aquarium? Just replace that gravel with some fresh gravel of the same kind. This guarantees a good batch of bacteria to start processing the wastes, and a much more reliable source than bottled elixirs. How long should you keep the "seed" gravel in the new aquarium? I keep it forever: I just leave it there. Unless there is a serious color-scheme clash, there is no reason you need to take the starter gravel out. If there is a color clash, I suggest placing the starter gravel in either clean pantyhose or a net of some kind and set it in the tank (preferably near the filter intake). Once your tank has cycled, just remove the bag.
Add your decorations, and, of course, the plants. Lots of plants. Although you want to have a well-planted aquarium for this system, you don't necessarily want to plant all of your plants at once. To start with, you should start with only the fast growing plants, such as bunch or stem plants like Rotala, Hygrophylia, Anacharis, or grasses like Sagitara or E. Tenellus (Pygmy Amazon chain), and the like. You should have between 60 and 70% of the surface area (looking down from above) covered from the start, and about 70-80% of that should be fast growing plants. The medium growers, such as Amazon Swords, can also be added now, or you can wait. Slow growers, such as Java fern and Anubius, should be added later, after the plants have established themselves; otherwise, they tend to attract algae.
Then, add a light stock of fish. In the 10 gallon tanks, I added 7 glowlight tetras (each tank)[FN 1] . In the 38, I started with only algae eating fish--three Siamese Algae Eaters and a black Molly.
Why do the plants help in this period? Plants don't just compete with algae for the nutrients in the water column, like phosphate and nitrogen. They can also compete with the bacteria for the ammonia and nitrite--both of which plants can utilize as well, particularly those bunch plants which process food above the gravel (i.e., not root feeders, but from the water column). Starting to get the picture? With the starter gravel and lots of live plants, you will have two sources hungry for the waste products your fish will start producing.
In the two 10 gallon tanks, the ammonia was barely measurable for the first 6 days, and it was never measurable in the 38G. Nitrites were never detected in any of the three tanks, and the nitrate level after cycling has been around zero (no reaction to the test kit). (See the accompanying chart).
One final note: don't just consider moving gravel from one tank to a cycling tank: driftwood and plants are abundant sources of bacteria due to their surface area.
FN1 : About 7 inches of fish. Since I also use two forms of filtration in these tanks, this load is just over 1/3 my maximum. You never want to exceed 1/2 of your maximum fish load when cycling a tank.

FN2: Table of Results (Using Aquarium Pharmaceutical Test kits)

Tank	Fish	Plants	Day1	Day2	Day3	Day4	Day5	Day6	Day7	Day8	Day9	Day10	Day11
10G	7 Glowlight Tetras, 1 Molly	5 large Vals, 3 medium vals, 2 dwarf sag., 1 bunch Rotala	0.1	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.0	0.0
38G	3 SAE	3Cabomba*(*bunch), 2 Rotala*, 2 Hygro. Polysperma1 Radican sword, 4 amazon swords, three pygmy swords*2 large vals, 1 italian val, 2 H. Difformis, 6 C. Cordata1 C. Wendetti, 4 C. bleri, Sag. subulata	0.0	0.0	0.0	0.0
	1 molly (added day 4)						0.0	0.0	0.0
	6 Raspboras (added day 7)									0.0	0.0	0.0	0.0
10G	7 Glowlight tetras, 2 Cory cats	5 large vals, 2 dwarf sag, 2 C. Parva, 2 C. Beckti	0.1	0.1	0.1	0.1	0.0	0.0	0.0	0.0	0.0	0.0	0.0
20G/L	6 Tetras, 1 otto, 1 Cory Cat	8 large vals, 2 sag subulata, 2 C. Parva, 2 C. Beckti, Hygro P.*, javamoss *	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.02	0.0	0.0

Other Sources:

Chuck Gadd's Article on Cycling with plants

Karen Randall--discussing plants and cycling