If I pour kegged beer into bottles and immediately cap them to take with me for the evening, will it remain well-carbonated?
I often do this with swing-top bottles. The carbonation suffers a little, but is fine for a day trip or evening out.
Put the bottles in the freezer an hour or so before filling. Warmer liquid does not hold as much CO2 and this prevents your beer from warming up upon touching the bottle. Fill all the way up and cap on foam. Pour and immediately cap, don't fill a few in a row before capping.
Read the answers to this question, too.
I recommend over carbing a little in the keg to help make up for losses. Chill your bottles down to help minimize the loss.
And don't pour!!! You can retrofit a bottle filler onto the end of some beer line and fill like that from a bev out connector.
You can also jam a peice of tubing like on a standard racking cane (3/8ths??) into a cobra tap. Make it as long as the bottles you want to fill and fill from the bottom up. This really minimizes carb loss and oxidation. Burp the keg and use only enough pressure to get the beer flowing (5-6psi).
Using a growler to take to parties is often the easiest option. Easy to fill, you probably already have a few around and will stay carbonated for a couple of days when filled from your regular tap. Bottling and capping is something I reserve exclusively for gift giving and competition entry and those recipes usually benefit from bottle conditioning anyway.
Yes! With a father that wants to try each of my beers, I often fill growlers directly from the keg after the beer carbonates (its just easy). As long as you "cap" it immediately and chill it, the carbonation will hold. I actually got a picture message last night of a stout I gave him 6 months ago in a growler that still had good carbonation. If you are planning on drinking the bottles that night....you'll be good to go. Hope this helps